Aw man I feel sorry for you.
You've got a hard choice do you retaliate and then give them a reason to do something even worse or standback and do nothing and maybe they'll think youre a soft target?
I used to hate dicks like that, I mean a bit of messing arounds ok I weve all done stuff like draw on people when asleep etc but when it starts getting to people that pisses me off.
Hang in there hope everything works out ok