If all else fails, take a series of sharp drill bits and drill until the head falls off. Just be careful not to break off the drill bit. That's why to start small and work your way up. You don't need to go larger than 1/4 because the stem of the bolt is smaller that that.
You should be left with a stub that you can grab with vise grips.
Failing that grind it off flush, centre punch and drill for an easy out, or re-thread if all else fails. Be careful, if the drill bit gets off centre and you get into the chinessium case, you are in trouble.