if you send them a letter back saying "i don't have a tv license, quit bothering me" they should stop sending you those letters... although i can't guarantee it'll work! i don't think it'll stop them coming round and checking, but at least they won't waste quite so much rainforest by repeat mail!
ps i'm sorry i've been really crap at getting organised this term, but i promise you'll get that coke at some stage!!! maybe week 1 next term, before all the madness hits?