Sometimes you just cant get these to stop spinning with tools from the top. The worst is you cant back up and tighten it at this point. Your committed to having to get this bolt extracted or your forks will be forever mushy in turns.
The last resort is to remove the front wheel like Richard said and grind off the head of the bolt and knock the whole mess out of the fork tube.
Then you can see what your going to have to do next.