Correction: There wasn't anything wrong with them, and now they have stripped extractor threads. Reusing the cranks will just make your or someone else's life harder the next time they have to come off. If you're ok with that, get them off using any brute force or chance-based method already mentioned.

The only sensible way to reuse cranks that have stripped threads is to repair them, like with the Stein crank extractor system or similar from Cyclus or VAR. The Stein tool works by using a guided tap to cut larger diameter threads, and comes with a special self extractor cap that threads into them.