That's my suggestion, too. Buy a minifridge--an inexpensive one, and spend the money you saved on a good lock for it (and for your bedroom door, where you can keep non-perishable food as well). Seriously. Do not leave any food where they can get to it, and if they ask about it tell them flat out that you're tired of paying for food that disappears before you get to eat it. You might be able to get a guy at the Home Depot or whatever to install the padlock on the fridge for you, and if not call around; there are lots of metalworking shops and/or locksmiths where someone would be happy to help you. (Assuming the fridge door is metal, because they usually are; a metalworker can bolt a hasp onto that metal surface, or even weld it on, depending on the finish, in about two minutes.)  Also, quit keeping their eating secrets for them. Tell the other about the fast-food-sneaking. You don't have to do it in a snitchy way, just a light laugh and a, "Oh, Doris was sitting right where you are eating that same thing yesterday," type of comment will do the trick. You don't owe them pretending they don't eat like hogs when they think nobody's looking.