When my cap was stuck I wrapped electrical tape around the jaws of some channel locks to keep it from scuffing up the cap and then I had a buddy hold the tube while I twisted the cap off with the channel locks.
It took some elbow grease and the jaws did cut through the tape and scuff my cap up a little bit, but I did get it to break loose and come off.
Did you try any solvent to help break it loose?