Roommate eats our food....?

My husband and I own a 3 br 3 ba home and have rented out 3rd room to a friend of a friends... We have a two year old who takes the 2nd room, therefore we pay 2/3 of rent and utilities... That's fine and all but since her and her fiance have moved in they eat our food! And I do not mean here and there, I mean ALL our food and just say they will "replace it". We buy 6-8 Gallons of milk a month so our son does not go with out, they drink 1/2 and "replace" it with A 1/2 Gallon! We have put meat in the fridge to thaw and come home to them finishing it up, LITERALLY saying I didn't know if you were going to eat it so we did! They pay 675.00 a mo and we pay 1,300.00 and then buy groceries which usually runs us about 300.00 but is now averaging about 600.00!!! I have tried writing do not eat notes, For this meal notes, and nothing has worked! My husband has suggested that I go to the grocery store daily and get only what we need for meals each day, but it is SO expensive and hard to do with a 2 yr old!!!! Any help at all or any pep talks or anything just gimme some feed back please cause this is getting old, fast! Thanx ahead