Some of you guys are lucky.  Compressing against the spring doesn't work every time for me.  Broomstick sometimes takes a couple tries even. 

I always use a 1/2" impact...  bigger gun works better for sure.

Good tools definitely make a difference, I always use my Snap On 1/2” Shockwave impact and my 60 gallon air compressor, it just gives the allen bolt such a impact it has no choice but to come loose, make sure your socket allen bit is a quality bit as well or it will either strip the allen socket bit or the allen bolt, I have Snap On socket allen bits, after lots of use they tend to get a little sloppy and rounded off, I turn them into the Snap On dealer and get a new bit installed, can definitely tell the difference in the snug fitment with a new allen bit.

I'll echo Lizeec as I stripped both a cheaper allen socket and partially stripped the allen bolt on an XS850 last week. Was able to get it loose with the fork in a vice, a better allen wrench, lots of heat and a few hard smacks on the allen wrench with a metal hammer.

Immobilizing the fork is significant to because you want the impact to be as sharp as possible, and holding or standing on the fork while using the impact muffles the impact.

Thanks.

OK, so I reassemble the fork with the top nut in and let the fork extend naturally uncompressed? Then use the whizzy gun?
I'll give that a try

Assemble fork with spring and top nut in and then compress the assembly as much as you can and hopefully pressure of spring will hold damping rod from spinning... 

Clean out the recess the Allen wrench goes into... even a few short blasts of brake cleaner... get as much meat as you can..
 Snap on has a "higher degree" of quality than the 5 fer a buck stuff... and eventually, you'll be frustration free and money ahead.. 
Sneezles61

     These are the forks in question

What I could do, is to insert some sort of fibre or cloth packing that will add grip to the spring/damper contact point?

I always just leave the forks together, put lower leg in a vise mounted horizonal with protective slip on clamps for the vise so you don't damage the surface or fork legs. Mine are heavy rubber. Take a impact gun with a good quality hex head bit (correct size) and pull down on the upper fork tube to put pressure on the damping rod or have someone push it in while you use the impact gun and it should come loose. 

Never had one that didn't come loose, but if for some reason that doesn't work you will need the correct tool to hold the damping rod. Maybe someone used loctite on the threads in the past? Big no no!

Can you insert a steel tube instead of spring then screw the cap in so it puts pressure on the damper but will need to be sorted so it compresses the topping out spring to get the benefit--if there's enough thread in the cap to do that. Ok, pull forks up & pull harder to feel the topping out short spring then slide your tube in & guess where to cut. Thinking tube will sit on the ledge of the damper top. Normally without all that, if you pull hard on the slider while rattle gunning the bolt it'll come out. Rattle it both ways to break it free. Topping spring gives some binding help. Must be loctite'd in, i don't do that.