Try getting out a pair of channel lock pliers and squeeze the cap enough to grab the insert that is just spinning inside the cap...after you get it off, you may need to replace the cap with a new one

Hi friendI agree with Jeffrey. You hve nothing to loose as cap will need to be renewed for sure. Take great care not to damage your tank in haste.If your still in trouble, drill two small holes in top of cap and partly drive in two screws. Grab these with pliers and slowly unscrew cap. Its a real last resort I once used myself.
Good luck