pull the bump stop down, clamp a vice grip on the shaft as high as you can (has to be above where the bump stop goes or you will wreck the seal!) and zap with an impact. if there is no room for a vice grip, use a rubber strap wrench anywhere along the shaft, it wont hurt it.

if you cant get to the shaft with a vice grip, *carefully* melt or heat up the nylon part of the nut and get it out of the way, no nylon=much less chance of it spinning.