Its all about their mentality than anything else, some people interpret student life as raiding cupboards, drinking till you get alcohol poisoning sleeping all day up all night.
Point here is that its hard to say what will work as its more about the mentality of the people than just saying this will work and that wont.
When I was at uni a few years ago I signed off jobseekers so had about £100 to last me about 4-8 weeks and stocked up on reduced food only to have it nicked, and the funny thing was it was nicked by a guy who had quite a bit of money, and he used to nick my food in front of 2 other tenants and share it with them and laugh about what a pushover I was, then again he even stole letters, and parcels for me, had 3 playstations sent in a package I had swapped with someone sent by courier and his bedroom had a clear view of the entrace to the point everyone had to walk past his window to get into building(and once when I locked myself out he didnt let me in and claimed "not to see me or hear me knocking/ringing the buzzer"
I think the simplest way is to intentionally buy something to waste i.e drink a little milk then leave bottle in fridge then when flatmates are in kitchen one day jokingly mention that your food is getting stolen so you left a bottle of milk in fridge with some spit and wee in it or something like that and see who freaks out, I did this with the 3 guys stealing from me and they actually threatened to beat me up and also SUE ME! for putting their health at risk then had the cheek to tell me ethey stole my food as I was greedy as I bought it in bulk and lazy for not going to corner shop daily!