Can´t remove BMX stem top cap bolt (won´t loosen) I was trying to remove the handlebar and stem of my bike in order to be replace them. The bike has got a BMX stem and the top cap bolt is spinning freely in both clock and counterclockwise.

Any idea on how to proceed to take it out?

Thanks in advance.
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.