I am going to assume that you have released everything holding the HS down (if not, try that first
).
Try using a hairdryer to warm up the heatsink, then pull up using a twisting motion to break the bond.
In the absolute worst case, place an old credit card under the HS and pry up using a flat head screwdriver.
Be careful what you pry against though; this is also a good way to break something!