(Sorry - I had to....)
OK, chances are that the stud is spinning and the nut isn't stripped, on the front wheel(if that's where it is) you might be able to get at the back of the stud from the rear of the hub and jam an old screwdriver or ??? in there to hold it enough to spin the nut off the other side (probably involve an extra set off hands) - just the first idea that popped into my head.
If it's on the rear, I'd hate to be in your shoes - but if I was, I'd try and split the nut somehow, possibly a dremel with a cutoff wheel or twelve (likely not enough room in there to wield a real grinder) ... The studs actually feed through the end of the axle so there's no real way to get at the back of them that I know of.
Then again, I'm a musician, not a mechanic - there may actually be an easier way that doesn't involve a torch - anybody else have a plan?