Do GMs let PCs kill other PCs?

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Does this happen in your games?

Do your players have their PCs willingly kill other player PCs for any reason (and we're excluding Paranoia games here for obvious reasons)? And do you, as GM, step in and stop it?

In an old AD&D campaign I was in, almost every character was evil or chaotic neutral. I was one of two good alignment PCs. I was the neutral good elven fighter/thief who sometimes sided with the bloodthirsty lawful good paladin. We worked together, but sometimes some folks were cut out of the loop out of selfishness or sacrificed willingly.

Which is not the same as killing another PC directly.

No, that happened in Champions, of all RPGs. Players would have their 'superhero' PCs kill other 'superhero' PCs because the concept was stupid, or because the player was bragging, or because of sheer spite, or because of sheer idiocy. Here are some examples with names and concepts tweaked to protect the guilty and remorseful:

• U.S. Secret Agent Alan Blackbird kills a new hero inspired by a Zulu warrior and a prophet (because the player doesn't like the hero's name and character concept). And so Shaka Jesus dies without having thrown a single punch, or even managing to turn the other cheek;
• The player of the mutant alien soldier Ranger is so ticked off at the player of electrical superhero Tesla Ivanovich's constant belittling of his character, and gets treated to a headshot through the eyesocket in the middle of the game, killing him instantly;
• Once again, Alan Blackbird kills a powerful fire based hero. This time, at the insistence of the player -- to show the other PCs just how tough his character is, he tells Alan Blackbird to shoot him in the chest. Alan Blackbird complies, but all Fire Laddie's defenses are based on activation rolls (there's a chance that the each of the defenses won't activate), and none of them activate. Fire Laddie dies, in game, to a sucking chest wound.

We always let these things lie, as the dice and the choices were considered sacrosanct. And the GMs in our group did too (though they would sometimes fudge if the heroes were acting particularly heroic when fighting villains).

In our defense, we were young and foolish and insecure and teenagers at the time.

How do you handle situations like this, if at all?