Removing a bolt...

[Blah blah]

I am looking to upgrade the forks on the kid's Skyteam Ace and there's a bolt in the top of one of the fork legs that I cannot move.

I can't push the forks through the top yoke to hold it so I can get some more leverage because of the way they're designed, and I did try dropping them a bit and just using the bottom yoke but that started to twist rather than the bolt coming off (something else I then had to sort). This 'bolt' also only has 2 flats so it's opened ended spanners all the way...

So how can I clamp a shiny fork leg without damaging it while applying considerable leverage to this nut? I've also thought about filing the 'flats' into a hex and attacking it with my rattle gun as that shifts most things, but I will still need to hold the leg still.

If I trash the nut then I'm going to have to get another made as I can't buy a replacement (I can't even get a complete fork leg!) but I only need to take it off once so a bit of damage won't be the end of the world.

I may be 'working from home' soon so could have a bit of time to sort it