had the same problem once, here's how I solved it:
reattach the bracket to the ski
unthread the nut, but leave it about half on
use an air chisel on the end of the bolt. the half threaded nut helps keep the chisel on the end of the bolt.
lift the pole up and down as you chisel
keep going until the bolt has backed out enough that you have to take off the nut, then carefully just chisel the bolt end. You'll probably have to follow the bolt through the bracket to remove it completely.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=47868 is what I used, with a pointed chisel.
either your bolt is bent, or the bracket is bent - you're probably going to have to replace one or the other.