go to homedepot and get a kriptonite lock, for locking ladders to ladder racks. it will fit thru the sproket and the rotor. it is a lot cheaper than the rotor locks they sell. if your rotor has big enough holes in it to fit thru.
me and all my buds have them on our bikes!
you can never be too secure!!!

 

if someone wanted to steal it they could just pick up the back and push it away.

 

hopefully you don't just use that lock to lock your bike! allways lock it to something that they can't just move. this is just another lock to add!