I'm guessing you shot many rounds thru it before trying to remove??
anyway, Let it sit. Not in your car, or holster, or rangebag... but on your table virtually drenched in oil. Leave over night.
If you believe you've soaked it sufficiently, get a THICK oven mit. Lock the slide back and place the mit over the thread protector. CAREFULLY and FIRMLY squeeze oven mit covered protector with vise grips and turn only enough to verify that you've loosend it.
YOU'RE NOT STRONG ENOUGH TO 'CRUSH' THE PROTECTOR WITH JUST YOUR FOREARM MUSCLES, BUT DO NOT TRY TO USE A VISE TO GET A BETTER GRIP.
THE 'NUT' IS THE PROPER THREAD, ISN'T IT???
THIS IS THE ONLY METHOD I'VE BEEN ABLE TO EMPLOY THAT DOESN'T LEAVE ANY DAMAGE TO ANY PART OF THE GUN (LEARNED THE HARDWAY)