If you have a Dremel or rotary tool, use a cut-off wheel to cut a slit in the top of the bolt, then use a flathead screwdriver to break it loose like you would any other flathead screw. Make sure you don't cut too deep, or you'll end up breaking the bolt, and that's not a good thing. It's very ghetto, but it was the only thing that worked when I managed to strip a cam retainer bolt on a car I was working on.

And the, of course, replace it!