yeh this just happened to me the other day put loads of oil/ wd 40 inbetween the headset and the nut and above the nut, put a wheel into the fork and clamp the wheel down so you don't damage the fork in any way, and then just get a wrench and just undo it - it might take quite a lot of pressure
Alright, thanks guys.
This shit has been pissing me off for like, 4 months now.
I just recently bolted it, (yeah!!) and then I was like "Hmm, let me try and get this stupid ass nut out." My hands are now officially sore.