The part that's spinning inside is the damper rod itself.
If you were able to break the bolt free with the impact gun, you can probably get enough force on the damper rod to keep it from turning by temporarily putting the spring and spacer back in and tightening the fork cap. If you're lucky, this will be enough force to keep the rod from spinnning while you hit it with the impact.
If not, you can try a broom handle/wooden dowel to hold the damper rod, buy the special tool, or just build one with some threaded rod and the right size nut. I have a post somewhere where I built one for diff bike you can do a search for to get an idea of what I did. Not sure what size nut you'll need for the B4 though :/