you could try something like banning (when you get opped in the channel - after filtering out before the [ you'll notice that there's the same number of characters in the nickname)):
//mode # +b *[??????!*@*
or (which might be more effective):
//mode # +b *!*XXX@*
or (if you can afford to do this):
//mode # +b *!*@*.ip
That should help for starters. Maybe you'd need to talk to somebody on your network about being join-flooded *shrug*
