How to remove a spinning bolt if you are lucky
Spinning bolts anywhere stink. This may get the bolt out if you are lucky, especially if Flash is correct and the bolt head is indeed broken and held by a few maybe damaged threads. Take a really small thin flat screw driver, and jam the blade under the head of the bolt and the washer. This cause the outward force required to back the bolt out. Use a small hammer and really wedge the blade between the head of the bolt and the washer. Then gently/slowly (counter clockwise) try to unscrew the bolt using a box wrench. If you are lucky the bolt will catch and back out. Or it may only move out slightly. Re-wedge and keep trying. Move up to a thicker blade eventually it may back out for you. Once you get it out you may have some options. Heli-coils, tap and die new treads etc, as suggested in some of the other responses.