You can use a "screw extractor" to remove it. They're sometimes referred to by their trade name "E-Z Out". To use one, you'll need to drill a small pilot hole, then insert the extractor and turn. The extractor looks like a pointed twisted cone, but you operate it by turning it counterclockwise. As it's turned, the cone embeds itself deeper and soon, you'll be turning the screw.

Sears makes an extractor that doesn't require a pilot hole. You just sort of jam this bit into the messed up head and turn it counterclockwise.

Of course, both these methods will mess up the head and you'll need a new bolt.

Perhaps can you jam something into the head, like a screwdriver and use a wrench to grasp the screwdriver shaft and turn?