If you can't lock your drawer, get a container (box, bag, whatever) with a lock on it and keep that in your drawer. Breaking the lock or stealing the whole container is a step up from stealing the food, at least in the thief's mind. They have no excuse for opening your drawer in the first place, so they can hardly complain about the lock.
(My assumption is that the thief has rationalized that stealing small amounts of food is ok, but stealing a container or breaking a lock is not.)
(I'm also assuming that you'd prefer to avoid a direct confrontation.)