Spinning Bolts....oh no

[limax2]

It sounds like the thread is stripped or the square nut that should be welded to the plate has come loose. Have a look under the plate at the nut and see if it turns when you turn the bolt. If it does, get a spanner or some pliers on the nut while you undo the bolt. The nuts are only welded on to ease assembly so you could just re-use it or replace it with a normal hex nut, preferably of the locking insert type.
If the nut is still welded on the plate then the thread on the nut or bolt must be stripped. You could try pulling up on the bolt while you undo it. If that doesn't work a last resort is to drill the head off.
Good luck with it.