The last time I had an obnoxious roommate, I made his life an utter, living nightmare in return.

In short, he'd leave his underwear sitting in the middle of the bathroom floor, toilet lid up, urine on the floor/walls/side of the sink where he'd missed in his "I don't give a Fart!" attitude...
Or he'd turn the kitchen into a disaster area, all to make something like a grilled cheese sandwhich. (Who uses a blender to make a GCS?! AAAGHHHH!)
Or, one of my favorites, he'd wake up at 3am, head out from his room in nothing but his socks, plop his furry butt down on my LEATHER couch, turn the tv on, and proceed to fart himself to sleep. (seriously)

So, in return, I started doing anything I could to make him want to leave...

Adding various substances to his coffee grounds to make him incontenent, constipated, hyper, sleepy, or anything else I could find from the chemist's across the street...
Or adding various puree'd vegetable slurry to his car's heater vents. (Nothing like the smell of boiled & rotting carrots, broccoli, and beats hitting you full in the face on an icy cold morning! Tee Hee)
Or, one of my favorites, taking his cigarettes (Hey, we're in an non-smoking apartment, it says so on the lease!), dipping them in cat urine, freezing them, and microwaving them dry to slip them back in the carton...

Once he'd angered me enough to retaliate, it took less than two weeks for him to get sick of it all and move out. (YAY!)
My next roommate was MUCH nicer...
For starters, she never left her stained & "racing stripe" streaked drawers sitting in the middle of the bathroom floor to the horror of Guests & Family alike... hehehehe