honestly.. id think you were being a bit of a dick...
your an adult, so are they - (despite how they may act at times) - just deal with it like grown ups, if someone steals your stuff. confront them (dont be passive agressive and leave notes, or bitch to others, this just leads to more tension), just go talk to them, deal with it head on.
honestly, food being stolen really isnt as much of a problem as its made out to be - ofcourse on sites like this, you have many many threads of people saying theyve had food nicked - but thats just a misrepresentation due to no one posting threads saying: 'yay! nothing got stolen!'.
the best thing to do, to avoid problems, have a good year, get on with your flatmates --- is just to go in with a really relaxed attitude. some people may drink your milk occasionally - but hoenstly, its not worth making a huge deal over, because there may be times when youl look in the fridge, be out of milk - and need some of theirs
but like i said, just go in with a mature attitude, and unless your quite unlucky, things will be fine - (in my flat in first year, no one had locks on anything, even the cupboards, and things were fine)