Been there, done that... carefully pounded (Pounding in the Allen head wrench actually helped break the threads loose) a bigger sized Allen head wrench into it to "create" new flats turned it back and forth slowly with lots of penetrant till it finally screwed off the crank arm......threw the offending cap away and replaced it with a new one with grease on its threads.....
Had one that was really seized onto the crank arm. Carefullty cut a slot on its face with my Dremel then used a big flat screw driver to twist it off. Threw the also mangled cap away and replaced it with a new one with grease on its threads and I was happy!
Although these caps can get a bit expensive, I just treat them as regular wear items, usually bunged up by previous owners of my bikes....
Frankly, I always left the caps of my crankset and usually etheir lost and tossed them out, years ago......but. but,..... C&V collecting......these days...