PC's only get as big as the boots you give them. Which makes for an interesting segway into the only time I HAD to kill a PC.
I'd started an Earthdwan campaign with soem friends at college and had been playing for a few months when one of them let this guy tag along to the game. Now this is a guy I'd seen around school, but he just gave me that weird vibe, ya know, that 'He frenches his pet possum' kinda weird guy vibe. That vibe was why I hadn't invited him to come play. But now he was at my house and wanted to get involved, and I didn't want to be rude, so we whipped him up a character.
Every time this guy would open his mouth, something stupid would come out. He would just giggle this dorky cackle and try to steal petty stuff from the party members. Just an annoyance. We tried letting the party sort themselves out, but it didn't work. Finally, he stepped on MY toes, my GM toes, in the middle of a bar fight he'd started. He just says "I pull a rock out of my shoe and throw it at the troll!"
What? Do you NORMALLY walk around with rocks in your shoes? Did you expect one to suddenly appear because you wanted it to? NO! just NO!
Next game session, he ended up trapped inside an underground chapel in a room full of alien eggs and facehuggers. Like the ones from the Aliens movies.A cave in seperated him from the rest of the party, so there was nothing they could do to save him. Sniff, sniff.
Remember, you, as GM, are GOD in every way the PCs are concerned. You can always tell them NO! and they just have to sit there and suck eggs. You control all the power. Don't let them play a character you haven't seen, they're probally trying to get away with something. If they have something that's getting out of hand, take it away from them. Items can be stolen/destroyed/damaged, Spells can produce side effects, abilites can be lost through divine intervention or illness. Or the best revenge of all, Make a copy of the party, The evil twin version of them, and watch as they destroy each other. When they players find themselves on even ground, they tend feel like they're being picked on. It's great.