Options for what to do if the star nut is spinning?
To resolve this situation, there are several options each with advantages and disadvantages (ie risks)
In my personal case it was a combination of multiple causes

The star nut hadn't been quite hammered home far enough, so the second line of &quot;leaves&quot; was still on the outside of the tube,
The tube was an aluminium handlebar that had been trimmed short.  So the internal wall thickness increases toward the center, unlike a bent steel handlebar.
The ID had been reamed out in the past to try and give more space for clamping the old rubber handlebar mount, but only to a depth of about 20mm.  The starnut had got to that lip and not gone any further.
Due to a turn in the bars, the mounting bolt was a little too long, and ended up threading through the starnut and bottoming out.  This pushed the bolt end sideways before it was tight, forcing the starnut back out a little, giving it less purchase on the walls and then it was free to spin and grind the inner walls of the bars without backing out.  I had locktite on the threads too, which didn't help the bolt to backout, even though it had not set.

Answer:

If you can't get the bolt to undo and you can hear the starnut turning, attempt to lock up the starnut by pulling it sideways, and twisting.   Assumes the topcap is not squashed down onto the steerer.
Another suggestion was to gently &quot;crush&quot; the outside of the tube so it constricts the starnut.  However this would be immediately fatal for a carbon part, really bad for an aluminium part, and only has a chance of succeeding with steel which will probably be permanently deformed by this action.  I didn't even try this.
If there's some level of bolt outside the top cap, you might get lucky by hammering it into the steerer, to help seat it enough to resist the bolt's undoing torque.
Decide what's more important or harder to replace, and use a drill to remove the other thing.  Drilling through the bolt cap would let you save your top-cap, or the mirror's arm in my case.
Once you can see the star nut then there are many more options.


Simply seating it further into the steerer tube will probably secure it nicely.
Do inspect the ID for scoring though - if its cut badly then this steerer and probably the whole fork is toast and needs replacing.
If its a straight steerer, you may choose to drift the starnut down and out the bottom of the fork.  Though this will likely ruin the starnut.
Lastly, you can use a drill bit and drill driver to shred the core of the starnut.  Once sufficiently thinned, the whole thing should fold up with long pliers and drop out by gravity.

Here's the bolt that had to be thinned and rethreaded to fit the starnut.  It was originally an M8 bolt for a motorbike.

Here's the starnut after it had been hammered home properly - there's a good 5mm of overlap now.  I also gobbed in a lot of epoxy to help lock things down, and then cleaned up the threads with an M6x1.0 tap.

Other options that I didn't try:

drilling through the bars to offer access to the starnut.  I felt this would weaken the bars even more right at the point where it was scored.
Trying to feed something in from the other end - with a steerer tube this might have worked, but mine was a wiggly set of bars with no real access.  And the other side had a star nut in it already.


One Month Later
The starnut failed internally after I whacked the mirror on a railroad crossing while negotiating a chicane.  The threaded core came loose from the two spring-steel &quot;flowers&quot;
I tried peening the flange to resist movement, but spring steel isn't malleable.  I considered epoxy but that was going to be hack.
Since I had a spare, the only fix was to remove the whole thing and replace.
I drilled out the core's flange, using a 9mm twist drill at an angle so the core wouldn't rotate.  Once they were sepatate, a 10mm twist drill caught the first spring steel flower and pulled it right out.  An M6 bolt in the core and a crowbar was enough to yank the the core out.  The inner flower got pushed well into the handlebar and is now unreachable around a curve, so there it stays.
The replacement starnut got scratched up with a file, and then I slathered the entire thing with 2 part epoxy before hammering home like the previous one.   The epoxy should help resist the rotational forces.
Future?  If it fails again, I will turn a ~20mm plug of aluminium, drill and tap for a M6 bolt, tap it into the bars and pin in place through the bars, under the bartape.  I might even contemplate using a left-handed thread for the main bolt.