Ball bearing in head cap bolt

I still want a picture

If they've used superglue then you should be able to use acetone / nail varnish remover to dissolve the superglue. Try putting some on a cotton bud, or soaking a piece of cotton wool or scrunched up tissue paper and resting it on it – they should keep the acetone in contact with the glue more effectively than just dripping it on. Avoid getting any on the frame as you'll probably damage the paintwork!

Can you afford to lose any of the steerer? If so - and if the other, better, solutions above fail - you could always saw through the steerer from one side to the other.

Can you access the other end of the bolt under the star fangled nut? If so, you could maybe use a socket on an extension to run two nuts against each other. Tightening the outer nut should drive the bolt up enough to loosen it from the cap. Then grip the top of the bolt with mole grips to get your nuts off.

problems with this approach-

maybe you can’t get in

you might get the two nuts stuck on

I am assuming that this is an A head and not a quill stem

Try drilling several holes in the top cap, near the bolt and not so far out that the steerer tube gets damaged, once these holes join together the top cap should break up and the stem can be removed and the forks dropped out of the headset. A pair of vice grips should allow for removal of the offending bolt and all that needs replaced is the top cap and bolt

It depends on the glue used, and the material of the cap.

I'd suggest putting a couple of drops of acetone on the glue, watching out to avoid drops and runs - if it's superglue (CA), it should more or less kill it.

Epoxies are, uh... Harder. A heatgun might help but likely won't, acetone and isopropyl might work but likely won't.
Taking a dremel to it and cutting a slot into the bearing might work - if nothing else, it might give you the leverage to cause it to pop out!

I have never tried it, but I thought it was a fairly well known anti-theft tactic.

If the seller did it correctly, he/she would have used super glue, and in which case, you have 2 options:

1) Super glue remover (should break down the glue - quick search on Amazon suggests you can buy it for less than a fiver)

2) Heat (standard super glue is not a great fan of heat).  Do you have one of those cook's blow torches, or perhaps a soldering iron?  Cheaper than super glue remover if you already have the necessary equipment.

If seller used epoxy resin instead of superglue, then you might have more trouble...

The other thing that can cause problems with this is if the ball bearing was too large for the allen key hole and got hammered in...

Good luck!

Drill two holes in the top cap and use a pin spanner to undo it, assuming there is enough friction to turn the bolt when turning the cap. If not, stick a bit of superglue around the bolt and try again.

Half-inch hole saw through the top cap, using WD40 to keep the cutting face lubed.

Could you unbolt the handlebars from the stem and unscrew the whole stem? If you pull up on the stem whilst unscrewing it there might be enough friction between the bolt and head cap to loosen it.

I'd probably go for a grinder with a thin disc and just cut vertically through it. Once the ball is out, you might be lucky and get the bolt out with a plain screwdriver.

You don't really want to be grinding aluminium, as it'll clog normal discs, so try and keep contact with any aluminium to a minimum.

I'd destroy the head cap with an angle grinder or maybe a small cold chisel. With luck you will save the stem and only need to replace the cap.
Did the seller only use this "antitheft "device for the stem, or are all the bolts similarly filled, to avoid theft of pedals, handlebars, saddle, etc? Which hole did he fill to stop theft of the whole bicycle? With preventative maintenance of this calibre, I'd be worried about any part that he'd fettled!

Totally want a picture of the worlds stupidest anti-theft device! 

I would say smack it f(carefully) with a hammer? Or heat it up?