Roommate issues

This one doesn't have a whole lot to do with the topics on here but, oh well.

How does one go about asking their roommates (who from day one said "we will not share any food items) not to eat their food? My roommates have been eating almost all the food I have in the freezer and fridge. I want to ask them without sounding like the bad person, yet again, about this topic. It's not that big of a deal if they replace what they eat, however they do not. How do I word it properly?