Idea: Cut off whatever you can and crack the rest off with a mallet. It *will* give eventually.
Another idea: If you have a strong enough drill bit, some lubrication and time with a decent drill will also go through the crankarm. It's up to you to pick the best spots where you think a hole will help it crack off. Suggested spots are the corners of the square hole. And please, be careful! Chainrings are annoyingly pointy when you're down there working with tools.
Last-resort idea: Stripped crankarm threads are probably the most annoying bike problem I've ever encountered. That said, others feel the same way, and well-equipped LBS'es will have tools to chase and possibly retap threads. They might also take pity and whack the arm off for you with heavy things, saving you some time and strength.
Check Barnett's for tried-and-true "just get it the hell off" tips, too:
http://www.bbinstitute.com/BM5%20chap%2020.pdf