Get a small dorm size fridge and put all your food (perishables) in there. Since your bedroom has a lock on it, lock up everything you don't want missing, such as drugs, food, ANYTHING you think she might want to "borrow". If she doesn't like it, too bad.

You might also want to label everything with your name. Like Craftlady said, maybe that's the reason why she's had so many roommates.