I'm going to lick my food that I know my roommates been eating
Why on earth would you lick that food then?
I'm going to do this because I know it will make them really mad.
Well, I think there are probably less disgusting ways to get your point across.  I understand that you don't want to have them eating stuff that doesn't belong to them, but have you tried just confronting them about it?
Nah, I'm not going to do that.
You are starting unnecessary strife between you and your roommate. If you don't like them why not just ignore them entirely?