Put a note on the fridge or email all of them:
"To the person who refuses to admit you're eating my food,
I have decided to stop playing nice. From now on, one item of food that you do not own in the fridge will be laced with one of the following:
- Something you're allergic to
- Extreme laxatives
- Bodily fluids (including but not limited to urine, feces, saliva, and sperm)
- Something you will regret eating after I put a picture of where it had been before on the fridge after you eat it
If any of you that are not eating my food are uncomfortable with knowing this is in the communal fridge and know who has been eating my food then feel free to confront them. Otherwise, continue smoking your weed and make sure to only eat what you buy."
On a more serious note, I would suggest cutting your budget even more so and eating things that don't need to be refrigerated or frozen, and if it is frozen/frigerated make it something you can eat day of buying it. Will require multiple trips to the store and a time crunch and mostly drinking water from tap, but hey- at least it's better than nothing.
jesus kirby. Noted.