h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The most effective way I have found of keeping what's food stuffs safe in communal fridges is to contract an unpleasant, reasonably life threatening illness (in my case pleurisy). Then go around licking items of food you own or drinking directly out of the carton. Now label the food as potentially hazardous. Most food thieves are not willing to risk illness. your food is saved. you find yourself not having to go out to corner shop when ill.

Please note: this method only works if you are not sharing bodily fluids with your housemates

The occasional random doping of milk with a powerful (yet taste-free) laxative works well (remember to post a warning on the front of the fridge), as long as you make sure you remember not to drink your doped milk yourself.

Alternatively, develop highly eclectic food tastes which few people have the appetite to match.