Some of the older designs have a threaded endcap. Make sure yours is not one of those designs.
If it's frozen on though you can try a little penetrant oil on it. I've worked on tons of them but never really ran into this issue. My guess would be that the bronze sleeve that runs on the axle between the freehub and the axle is possibly wedged or jammed in there for some reason.
Was going to mention tot he OP to be sure to check that the bronze bushing came off/apart from the freehub.