1. Dorm fridge
2. Talk to them before being a pussy and leaving a note.
3. If they don't listen after you talk to them and leave the note, the next time you make food make it with toilet water. Once it's been eaten, leave another note explaining that you don't know where your toilet water food went.
4. If they continue to eat your food, upgrade to laxatives. Don't leave a note that time though, because it's HILARIOUS and worth the money.
I've tried this with putting notes, but it never works. They just get weirded out by the passive aggression, like you're some introvert autistic guy who can't express his emotions. Roommates and students have a bad diet regime, so they'll eat whatever they can find.
Don't leave a note. Man up and make threats of physical violence in person. Better yet, you should just greet your roommates with a punch to the groin when they come home. Another idea is to buy a shotgun and leave it by the fridge. Don't say anything about it, just clean it every time they're home. And by all means, if losing your shit means posting on GAF instead of directly approaching the problem, have at it!
My roommates and I agree'd early on that we could eat each others food as long as we leave enough for the person that bought it to have the last bowl/dish of whatever. Works well, no one eats more than a bit of something here or there and things remain copacetic.
I really dont get the concept of living with strangers.
I know you gotta split the costs and all but Id rather be super poor and struggling than have roommates I dont know. I know a single mother living in a small condo with her 6 year old daughter, and she has a car. If she can do it so can you.
But it's possible--if kind of thoughtless (in the literal sense) and inconsiderate--that they aren't aware of this. Maybe they think it's ok to share everything and it will "even out". Again, that's a little dumb but not necessarily malicious. But leaving a pissy note is definitely dickish and passive-aggressive.
The point is you can't get really righteously angry until you've explicitly explained to them that you don't want anyone touching your food. In person.