Quick
Tips on how to passive aggressively stop your roommates from eating your food. GO!
EDIT: What about frozen food?
I suggest that you put something gross in place of something they typically steal. For example, if they usually use your milk, put spoiled milk in a container with a good expiration date. (Gotcha! You thought I was gonna say semen)
If that doesn’t teach them, at least you got some good old fashioned revenge! Best served spoiled MmMmMMmm