Oh those things can be a nuisance.
I assume the damper Bolt just spins?
If thats the prob,,
I further assume you have the Fork Springs removed?
I cheat---I use an air wrench,,,the sudden torque and sheer speed never fails.
At home without air tools,,,it helps to leave the springs in the fork ,,with fork caps on.
The Damper Rod is "the lower spring seat" inside the fork leg,,,
so the spring load helps hold the damper rod from turning while You're trying to Undo the bolt that holds the fork together.
Another "cheat" if you already have the springs out,,,is to stick a mop handle down the fork,,,and PUSH HARD.
That will put enough friction on the damper rod to hold it from turning while removing bolt.
Most of the time,anyway.
Best way Really is to have the spring installed.
Don't worry about the fork hand-grenading when you remove the bolt "while under Spring Pressure".
It's NOT under spring pressure,,ever.
The Damper Rod bottoms out in the fork leg in fully extended position.
When you Compress the Fork,,,the Damper Rod goes Up Into the Fork Leg.
Then when it rebounds//Fork Exntends//...Dampre Rod comes back OUT of the Fork,,but only so far.
The OUTER fork tube is bolted to end of Damper,,,all it does is ride along with the damper.
The Spring is trapped inside the Inner Fork Leg,,between Cap & Damper.
Nothing comes flying apart when ya remove Damper Bolt,,except Fork Outer Tubes "fallin off" by gravity
Actually,,they are stopped from separating by Seals/Upper Bushing.
Try re-installing the springs,,,then try the bolt again
When ya go to Reassemble,,guess what?
Same Deal in reverse.
Ya Gotta Hold the Damper Rod from turn while installing & tightening bolt.
Air Wrench works like Magic,,,,otherwise The Springs being installed put enough bind on Damper Rod tp prevent / minimize turning while Ya get Bolt tightened.
Hope that makes sense
That bolt is a silly-simple thing,,,which only makes it Double Annoying when it wont cooperate
Good Luck