The thermal epoxy will become brittle at near freezing temperatures. Throwing it in the freezer for a while and breaking the heatsink loose should be effective without causing any damage.
+1. I do this with heatsinks all the time. Put it in the freezer for a while, then grip it with a paper towel or something and twist back and forth on the heatsink until it comes loose.
All I ever wanted was an honest weeks pay for an honest days work.