Not that I would recommend this but it may be a possibility. You could probably try to remove it with the computer on. The heat generated by the cpu may soften the thermal paste enough to make seperating the cpu and heatsink a bit easier. Of course I wouldn't pull too hard, but if this method does work then definately shut the computer off as fast as possible to prevent overheating the cpu.
Or you could try using the heat from a blow dryer instead, but be careful not to use it too long and melt anything. Of course this idea may be worthless if the thermal paste has dried to the point of becoming like cement.