Take a big dookie in a paper bag, light it on fire, put it in front of his doorstep and ring the bell. He'll stomp on the doodoo trying to put out the fire, it's great.

Or you could find a recently dead animal (or painlessly kill his family pet) and mutilate it almost beyond recognition before throwing it through his window. He'll think you're a psychopath, it'll give him the spooks, especially when he starts noticing you following him

Also find out where he works, confront him at his workplace in front of his boss. Be sure that the whole 'you stole my drugs' part is loud and clear, hopefully it's a service industry and customers can also be audience

Rip the metal folding door and flag off his mailbox just because he's a faggot and doesn't deserve that shit

Tell his parents. Tell his parents. Tell his parents.

Call the cops, anonymously snitch your ass off. Maybe even include the parents and plant drugs on them if you have lots of free time, just to make the situation a whole lot shittier

Main thing though, find some friends, corner him, and beat him. He won't go to the police, mark my words