Can´t remove BMX stem top cap bolt (won´t loosen)
It could be that either the star fangled nut itself is spinning or the inserted threads in the nut are spinning.  If you take off the front wheel and look up the steerer tube can you see the nut?  Is it spinning?  It it is spinning just stick a long screw driver up there to hold it in place while you take out the top cap bolt.  You can hammer in a new star fangled nut on top of the old broken one.