Those bolts screw into a 'captive nut'.. which simply clips into the frame bracket. The clip portion of the captive nut is much thinner metal and can rust away before the nut does.. and when the clip (that holds it in place) is rusted, then the nut (and whatever portion of the clip that remains) just 'spins' when you try to remove the bolt. As someone said, remove the head of the bolt (without damaging the other parts) using a cut off wheel or hack saw or chisel?).. and go from there..