Brake axle nut thread gone

Hi Everyone,

Put a brand new (budget) front brake onto my fixie, that I couldn't tighten properly, it was sitting loose in the fork. Kept trying and probably damaged it, so now I can turn it both directions, but it doesn't tighten or loosen at all even if:

I take the wheels off and hold the nut with a wrench, whilst turning the brake itself around.
Holding the axle and turning the nut.
Wedged directly behind the nut putting pressure on and trying to unscrew it.

Any ideas how to take it off the fork? Thanks guys.