My room mate keeps going in to my room and taking my stuff. I need some way to prevent this until the university can move me in 3 months. I asked the school if I could just replace my current door knob with a locking one but they said no. this I would need some sort of cover to go over the external door knob that I can lock. Do you know of anything like that?

They are called door knob bumpers and usually available at stores that sell baby and/or bathroom items, like Bed Bathe and Beyond.

Chuck

If he is your roommate, he is allowed access to the room. If he is taking things of value, call the campus police.

I may have missed part of this. If you have a separate door to your room but a common door to the apartment, I'd change the lock to your room.