Yes, I know that there are currently two other threads on getting back at someone, but this is so brilliant it deserves its own. Because A. its extremely simple, and B. it fucks the person over for like months, especially if it is a girl.
Ok, so all you do is get a bottle of that Nair instant hair removal stuff, and when you're at their house, use their bathroom, dump out their shampoo, and put the Nair in it. When they go to wash their hair, instead of just getting clean, it will all fall out. And then they're like Oh Shit im a skinhead!!! and hair doesnt grow back quickly, so they're basically fucked.