if you are dealing with a steel 1-1/8" steerer then there is a simple solution involving (ahem) a big hammer....
Get a length of tubing (ideally steel) which is just small enough to be inserted into the steerer from below. 7/8" or 15/16" OD tube is usually about right; the steerer bore is nominally 1" but the bottom of it may be distorted from welding or have burrs from being drilled.
Invert the bike, place the tube in contact with the underside of the SFN, rest the top end of the steerer on a piece of wood (with a hole drilled in it, big enough to accept the M6 bolt), and simply start boshing away.
The lower part of the star will soon release its grip and in fact at this stage it may bind onto the threaded centre well enough that you can undo the bolt like you should be able to.
If this does not happen then continued boshing will do one of several things;
-it may collapse the lower star so that the tube bears against the upper star and releases that too
- it may cause the upper star to start to slide in the steerer
- it may collapse the upper star anyway.
If the bolt comes out and the upper star is still gripping the steerer, it can usually be pryed one petal at a time using a screwdriver so that it loses its grip.
This method should not be attempted with an aluminium steerer; you will just damage the inside of the steerer too much.
If successful this method leaves no junk inside the steerer tube.
cheers