Does this uni have a resident housing team? They are usually a port of call for if you have to confront someone you live with and are possibly struggling.
From my experience, "acts of revenge" never help in this. Trust me, i tried them, it just makes a bad situation worse. Doing nothing wont help if you worry this much about it (been there too). Your best bet is to confront them about it. Maybe it was an accident and they will stop if they realise how serious you take this to be. I lived with someone who wouldn't give a damn about taking other peoples food, and it kept getting bad until we all confronted him about it.
Another benefit of bringing in your housing support team, if available, is that they might be able to help you move, if thats the only option. Sometimes if people hearing that, it can make them stop. There may also be some sort of punishment the housing support team can make either, though i doubt it.