It sounds to me like the nuts or nut plates inside have become detached and are just spinning on the bolt. You are probably going to have to drill out the head and get the post off to see whats going on in there. If you start out with a small bit and work up to a larger one you should be able to remove the head.

As for a repair once that is off, maybe a nutsert or perhaps you can get in there to weld a new nut in place.

Don