Take a papertowl and place it nice and flat in the freezer. Then place the vid card onto the papertowl. Wait about 60 seconds and take the vid card and papertowl out of the freezer. Now put an edge of the paper towl under a corner of the heatsink, take a flathead screwdriver and use it to break off the heatsink. It will make an unpleasant sound, but don't worry, that is the epoxy giving.
The reason for the freezer thing is because the epoxy becomes brittle when it's cold. This makes it break as easily as possible. Now use some acetone or nail polish remover to eat away at the epoxy that is left on the GeForce's core, and use some low grit sandpaper to sand it away (don't sand too much, you might dig into the ceramic coating on the core!). The reason for getting the epoxy off is because heatsinks require flat surface to be able to dissipate the heat properly. Good luck