there is a tool to try and re-crush the rivet into the subframe. or, you can use a homemade method. try to thread a bolt into the hole as far as possible, putting an extra nut on the bolt before it goes into the insert. if you get enough purchase on the threads, you can spin the nut down to the suface of the insert, and tighten it holding the bolt from spinning (then run a tap through it). this will re-squish the insert, and maybe it will quit spinning. maybe not, then try 609 loctite, very carefully.
mo