Okay so I will never tell you what I've done done.. But I'll tell you what one of my homegirls did when she found out that her roomate was having sex on my friend's bed when she went home for the weekends.
This one day she came home and found "stains" on her bed so she waited till the roomate went to the bathroom and urinated in her cup of Ginger ale... when the roomate came back my friend watched her drink it!

NOW THAT"S GANSTA!!! * and nasty... but it's revenge.