Although I don't have much experience with threadless headsets, I do understand how they work. But here's the problem: my wife's hybrid, which does have a threadless headset, developed some play (just a little) in the headset. I went to tighten it in what I think is the usual way: loosen the pinch bolts, then tighten the cap bolt enough to get rid of the play. Didn't work: the cap nut turns freely, but is evidently stripped--where it threads into the star nut, I'm guessing. The solution should be to remove the cap bolt and cap, drive the old star nut further into the steerer (not a carbon steerer or frame), and thread in a new cap bolt.
The problem is that the damn cap bolt won't come out. It turns freely, but I can't think of a clean way to get a strong enough grip on it to remove it. I tried all the easy obvious stuff, like pulling up on the (loosened) stem while turning the bolt, gently prying up on the cap with a screwdriver, etc.
This can't be an uncommon problem. Any ideas? Sacrifice the cap bolt by drilling a bunch of holes around the bolt head? A dremel or rotozip-type tool would make that easier, but I don't have one.
As an aside, I'm kind of confused as to how this bolt stripped in the first place. I had it out a year or so ago to install a stem extension so V. could ride the bike after some shoulder surgery; after she healed I took off the extension and reinstalled the stem directly on the steerer, adjusting it correctly at the time--pretty confident I didn't overtighten it. I have my faults, but it's been a long time since I overtightened and stripped a bolt.

Sounds like a more Modern Issue, though I do have ONE newer bike with threadless, I have NO clue as to how to attack your issue. Perhaps over in the Mechanics Area you will get more response.

Yeah, maybe, but the mechanic's forum doesn't seem like such a friendly place to me. It seems like a lot of the C&V types have some modern bikes, too.

Well I don't see a problem with sacrificing the cap since they are easily replaceable. I've never seen this problem before...

Yeah, just sacrifice the top cap it's a cheap part.

Can you jam something against the bottom of the star nut? Insert a long screwdriver or equivalent from the bottom of the steerer tube.