Acetone should remove the last of it (nail polish remover is acetone-based if you happen to have any) and then use alchohol to do the final wipedown, as nailpolish remover has other additives you'd want to clean off. You shouldn't apply any other compound until you've gotten all the visible thermal pad off.
Another option if you can remove all the plastic from the heatsink (fan, shroud) is to put it in an oven or toaster oven for a few minutes at the lowest temperature setting (150F works), let it get really warm, and the thermal pad becomes almost runny, easy to just wipe off.