It could be it hasn't been off in a long time. Assuming there is any "stub" left to grab I would expect a big pipe/monkey wrench would grab it pretty good. You might try it cold and then try it hot. Might work better one way or the other. Just be aware that you could (technically) damage the pan if it is on really really tight! With a big enough wrench something will give eventually... I will be interested to hear what finally works.

Mike