That depends on the ircd, but on Freenode you could decloak using the services to ban the user from channel using IP ranges and see if they trigger (I think. It has been a few years since I this trick used, and it might've not even been bans). Extremely inefficient, but if you have a bouncer with auto rejoin on...

In any case there are some really obsessed people who probably know a hundred ways to take over your channels at the very least. Seeing some of these people do their thing is one of the reasons why I steer clear of IRC these days.