You have to start thinking at your roomie's level. Start marking your stuff that you own, or label it. Put price tags on the stuff too. Turn your kitchen into a super market. If they eat something they will have to pay the marked price and it will be deducted from the monthly rent bill. For every trash bag you have to take out to mark off a few bucks.
You really have to assert yourself here. They are totally taking advantage of you. I think they are still in the "leeching off my parents" mode where they take stuff from the fridge as if they were at home. Make sure they get the message that the stuff they are eatting costs money and they need to pay for it.
I don't have any problems with my roomies with food or anything like that. I have my own room and restroom so it's up to me to clean it up or leave it as messy as I want.