...do you feel obligated to stop it?

Example: Something described as dangerous, unsafe, generally a bad idea, making it clear that PCs will probably die if they do it...that they then decide to do anyway.

Do you keep hammering home that it's a bad idea repeatedly, even though no one is listening, or is there a point where you just throw up your hands and go 'All right, if you want to commit character suicide, go ahead'?

No. It’s up to the players to make choices for their characters. If they choose to throw away their characters, that’s their choice. Signal things are bad news and dangerous, but it’s on them to engage, run away, etc. Let the dice fall where they may. If the players are too stubborn to run, they will learn the hard way that running is almost always an option.

across  all three of my games, I try to let the   players know, that ...yes...very bad things   can hapen  if   you chose the wrong path.

BUT..some people like to choose the wrong path.  I have NPCs and other 'tips" that will  color the way a bit.. But if  i just say " you do this. you'll die"..why did i give them  the choice.

 Over the years, there  were really , really dumb things, that the Players were  warned about  in some shape or  form, and  Characters Died  or things went very badly.. That's the way it goes. There Outcomes are there, to happen, but only  if the players   push their characters into  those spots.

I lost some players because of  very bad  outcomes... i also had players   step up  and  try to recover from what  had  happen, like a story should be. Had a people  whose characters  died,, and then   came back   with a new character  ready to go..while  a few  just walked away after  rage quitting ect ect.

 so?  If you don't want to have them go into a place that is 'bad'..then you, have two choices...something 'happens" they cannot go there.. or..you just  don't  set that up.

For My part. I was in a game  on  a now  , dead site. My Fav  character.  helped  'lessen' the damage  of a 'red  Wedding  ' type meeting ( this was a decade or  more  before GOT ) ..anyway..My character went  back to recure  another one, fighting his way through bad  guys...The  other  PCs   decided she  was going  stay  with NPCs... the  'delay' in my PC getting back to the people he commanded, would end up causing his death... I was crushed...But , I broth  in the 'back up character..and the site died... i now play that game on a ..not very good  site( in my opinion...I tried to get them to come here)..and 'stories  are passed along about  my first character .

so , yeah? If the players play the games  as they should be played, they have to suffer  the  blowback of making a poor choice. That? Can  tell you straight out, how good  a player you have.

There's a lottery slogan where I live that I feel applies well to D&D: "know your limits, play within it". If you want to push those limits, that's fine, but you know what the potential consequences are. If you willingly go ahead with something described as dangerous, I let the dice fall where they fall. One time that meant the threat that could kill each member of the party one after the other with ease failed every save and missed every attack. More often than not it means the graveyard gets a few new headstones. If you've been warned about the potential for a TPK or that the situation might be over your head, and you choose to go ahead, that was your choice.

As a player, sometimes it seems the character would do it anyway, knowing how dangerous it is and likely they are to die.  Some people are like that in real life.Willing to try because of the possibility for success, despite the probability of failure.  I always ask myself, "What would my character do, if given this choice?"

I think that knowing that the death of your character is something that can really happen makes your victories sweeter. If you know that anyway the GM will save your ass no matter what, the game becomes less challenging and probably less interesting.

  there has to be  risk?  I mean in somewhat serious games. My games have banter, but   no outwardly foolishness., its like a Movie  or good book. The  motto  i give  folks in my interviews is this " I am not a character Killer, But if you walk into the dragon's  moth to get the coin in its teeth, you're gonna get eaten"

and? that just doesn't mean death. There are lots of bad stuff   that  can happen that aren't death.

And that's the truth.

I write a bit (Unpublished, semi-fan fiction, but my readers love it. Both of them.) and after creating a protagonist / antagonist you need to have them act according to their character or establish a really good reason why they didn't. For me it's equal parts fun and frustration when this happens.
Likewise any character I roll up for a game follows the same ethos. They might evolve over time, but having them change from say 'Reckless and impulsive'  to 'Cautious and Thoughtful" between scenes just because it's the best way to play the game is something I'm not going to do without a lot of fast talking and fancy footwork.

Deliberately breaking character in order for a better outcome is poor Roleplaying from the Player. "My Dwarf character hates Elves, but they're going to be nice to the Elf Queen as she might give us a magic doodad."

If the GM knows a Character (or party) almost always act in a certain way, putting them in a situation that will result in their death unless they break character is pretty poor on the GMs part. "Your character always kicks down locked doors, so I'm going to trap the next door with a massive bomb."

But as the GM is God of the game, I tend to place the onus on the GM to ensure that if this situation ever occurs, they provide some way out provided the Player isn't a complete moron. However if the character is written as a moron, things can get really interesting; I had this once where I played a character who was cool as an iceberg and about as smart, the GM set a situation where my characters reaction was..unwise but utterly predictable and not behaving in such a way would be out of character. Ultimately it cost him his life (which I'm still a bit salty about as two other PCs decided the best way to deal with my character was to kill him and the GM let it happen), but he was still in character when the bodybag was zipped up.

Stop it? What's the fun in that?
ANY situation where certain death is imminent makes the governors pop loose allowing characters to do that one thing that's ultra-powerful and suicidal at the same time. The terribly dramatic overkill action (we all try to build one into our characters, admit it) that couldn't be used because it kills your character is the ONLY worthy response to such a situation.

To borrow a quote: "It seemed like a good idea at the time."
Which goes right next to: "Hold my beer and watch this."

Humor aside, I'd say that the answer is ultimately going to be "no".  If the party of 3rd level adventurers want to torque off the ancient red dragon and get eaten...well, see above.

the whole thing here, for my point of  view. If you have a comic game, were people are just screwing around..yeha?  let them ddo stuff , get squashed like grapes  ect ect.

But , if you are in a Living world game, where  characters are supposed to be growing, you need  to warn then, and...again..let the chips fall where they may.

 Most of my players  really  are into the games  and the  building characters.. some  times  a new  player  comes in and   does 'stupid stuff"..that  rubs folks the  wrong  way.

 so?  you run/play the game as you will. The Gaming gods  will take it from there.

There needs to be consequences, especially for deliberately bad choices.

However, a TPK is undesirable as it ends the game.

A GM can imprison them,  remove their possessions, reduce their health, put them in debt to powerful malicious beings, cost them social favour points, cripple them for a significant amount of time.... and the GM is being nice. Hey, he could have killed them.

"You wake up in a rough shaped tunnel of black rock with something glistening and sticky oozing down the walls. The mouth of the cave ends in rusty iron bars, and you can see a dragonborn standing guard a short distance away."

"The last thing you remember is battling a dragon and losing, the rogue going down, the mage falling from midair, the fighter last man standing and dripping from wounds. Then the dragon breathed, a noxoious green cloud of gas, and gasping and choking you passed into unconsciousness."

To misquote an old aphorism:

TPK us once shame on you,
TPK us twice, shame on us.

Bad jokes aside, it depends on context.

Ultimately, TPKs aren't exactly a goal you should be aiming for as a GM, if nothing else to avoid the hassle of putting together a new party, and if there's a chance that the TPK is your fault as the GM (i.e. you've either deliberately set up the party to fail, or there's been a convolution where your players have got the wrong end of the stick and are making decisions based on flawed information that they couldn't be expected to know otherwise), then yes, you should look to avert it by the best means necessary.

That being said, if you've warned a party again and again for doing stupid stuff which doesn't fit within the genre of game you originally agreed to run, then there's a point past which they need to see the consequences of their actions, and TPKing can be a blunt tool to basically say 'either stop *banana*-ing around or go and play a game which fits the level of serious you want to play at'.

Again, I'd still find a way of doing it in a manner which gave the players once last chance, so basically offer to magically whisk the party away at the last minute if they don't want to role new characters, but make it very clear next time, you don't get a do-over.