In my case it has to be a group decision. It happened once, or twice, but also we had a few cases where people just didn't fit in, and we didn't have to tell them that. As for both aforementioned cases, one player was being a dick to guys in the group, and couldn't refrain from hitting on the girls in a very uncivilized manner. He also boasted 20+ years of experience, but obviously the concept of player growth was lost to him - he never went beyond killing stuff and screwing NPC's. So after a few failed attempts of communication we gathered and made a decision... after like 15 minutes. I had to play the devil's advocate, but despite doing it properly, even if against my own judgement, the vote was unanimous.
The second case wasn't that easy. The girl who had become a real problem was a decent player, and a nice enough person. But something in RPG's ticked her the wrong way, and her characters were all backstabbing, scheming and plotting against the other player characters. Not all of them - only those who didn't follow her blindly. She managed to convince two of them that they were a team, and it kinda went downhill from there. So in the end I called for a split. I even went as far as finding a GM for the 'team' of hers, an acquaintance who loved that playstyle, and fancied himself a good Vampire GM. In short: we managed to find a nice pair of players for our team, and our problem ended up marrying her new GM after a while. I really hope they won't breed, as that will take a toll on my karma... =='
P.S. I am with Critas on that one - that was way more immature and rude than what the kid did. You could've given him the talk, and help him get over all that stupid crap he had in his head. Instead you've just deprived him of the chance to learn something.