(Just lost my first (longer) post due to losing signal-bugger!
ARGH, I hate it when that happens. I've got into the habit now of copying to the clipboard before posting with long posts as a precaution for that.
I'm not 100% sure because like I say I not taken it off before. But it is going to spin by the looks of it as it's on the other end of the crankshaft (opposite to the flywheel) and done up to 75ib ft, so pretty tight. Your going to need a torque wrench or at least a good ratchet or breaker bar. Torque wrench would be best as it can be done back up to the proper torque.
As for locking it off and stopping it from spinning, putting the bike in gear and having someone stand on the rear brake pedal may work as that should stop the clutch spinning, as if your removing the the front sprocket for chain replacement. Looking on page 9-22 in the workshop manual the primary gear/sub gear is tied in with the clutch basket so in theory if the clutch does not turn the primary drive/sub gear should not. Or there is a round hole on the gear so maybe a screwdriver through that and against the clutch basket may lock it off, or even something directly between the two gears. Again 9-22 in the manual shows where the Honda tool (££££££) Gear holder goes between the two gears. It's probably not as difficult as it looks, just be careful of those springs on the primary drive/sub gear and don't remove it. Fit the new advancer and threadlock the bolt before torquing back up.
I would prefer not to use something metal between the two gears as to not damage the spurs on the gears. I would attempt stuffing some clumped up rag between them or

a rubber block. An old bit of rubber from a broken cush drive would probably work well as it's tapered. It would work just as well on the top to stop it spinning anti clockwise when undoing or on the bottom to stop it turning clockwise when tightening back up. Saying all that though, if putting it in gear and having someone stand on the brake works, that would be a lot easier.
Anyway that's my conclusion from looking in the manual. Lets see what Tony has to say, I'm sure he has some ingenious solution. Or even Benny.
When are you getting the Jet kit installed mate. That's what really interests me, I'm curious on how drilling the main air jets is going to affect things and what size main jets you will end up with.
(:-})