My last two experiences:
1. The drug addict. This guy even brought his meth pipe to the game. He took constant cell phone calls right in the middle of the action and was always 'up to something'. When the pipe showed up he was showed out.
2. Complainer: Complains about EVERYTHING. The DM style is wrong. THe player's style is wrong. His character sucks. Your character sucks. That book sucks. This book sucks. His life sucks. Your life sucks. The room sucks. Outside sucks. THe coke is bad. The pizza was no good. That movie sucks. These dice sucks. The NPC sucks. The current plot event sucks. etc... He'd also take players aside and try to convince them of how everything sucked. I DM'd him, then played with him. It was frustrating on both ends. He managed to rip two campaigns apart by getting both DMs to feel unappreciated and frustrated at every angle (after all, everything we did sucked, and the other players were all roleplaying it wrong)...
The last one before this was 'The AD&D Player'. Shadowrun game this guy brings the dungeon crawl mentality to the game. He was overly agressive and rules exploitive. Accussed the DM of being a 'munchkin' for demanding that he balance everything onhis character to match what the rulebook says it should be... Started gunning for the other player's PCs. STarted getting a little agressive 'out of the game'. Would cause big arguments when told he couldn't use 'unbalanced suppliment X' from the old edition of the game... That game also had one 'man in black' (wear any color as long as it's black, spend all day talking about depressing goth stuff cause it's 'artistic').
I try to screen; but one or two always slip through the cracks and I have to edit...