I have a lock on my rotor you can't cut with a bolt cutter but that doesn't stop them from picking it up and carrying it away.The one I use looks like this and fits my rear rotor.
http://www.masterlocks.com/Models.asp?Target=No.%2037%20Padlocks

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!