Offer to take his stuff to the laundromat because you dont have much to wash so dont mind fitting theirs in the mix. Make sure you've seen Roomie in their more expensive gear recently so you can get that in the pile. Show your clothes (aka some homeless dudes stuff you got for $2) and explain how its your most pricless piece. Come back 2hours later with nothing - say someone stole "our" clothes.

To add to insanity though, wait until roomie is away, wet their clothes in cold water then run them through a dryer at hottest possible temperature. Put them back how you found them, only shrunk a few sizes smaller. If not up for shrinking, wash their work clothes with cigarette butts and other stinky shit from the street, put it back before they notice it was gone.

Or just fill your room with camphor, then release a bunch of moths in their closet.

Invite friends over for a party when you see Roomie has the fridge and pantry well stocked. Tell your mates to eat everything, take stuff home for themselves. Or just say you had a party and clean out. When Roomie asks about it, say they'll have to ask your "work mates" because you made clear it wasnt yours to take, and these people will own up if they did it: Give him random large-business phone numbers to call for "Tom" and "Sally" to pay for his milk.

Or just lock all your pantry items in your room, storage tub, then release pantry moths in the kitchen. For more insanity, just put random dead bugs in their food.

Snip ceiling light wires, fridge wires, any wire with enough slack to strip and rejoin later (remember to flick off the fuse first), and remove parts from anything mechanical (doors, toilet, chairs) - all stuff you can fix for $0-5 or arent affected by, but will cost Roomie a full trade repair job to figure out.

Hide Roomie's key or replace it on the keyring with a matching dud key, play dumb. Don't answer the door when they inevitably bang on it to be let in, pretend you're asleep.

If they have a car, thats a goldmine for shit that'll make someone go insane.