Do this.
1) Get Anti-Static bag, large enough for CPU+HSF.
2) Seal it up. Loads of tape just to be sure.
3) Place in freezer for an hour.
4) Take out of freezer, and remove HSF from CPU.
Notes:
May require a little longer than an hour.
If it doesnt come off easy, then give it a bit of "persuasion".
If you f#$k it up, dont blame me.