just ban whatever comes after the @ in the whois, EG:
name!ident@address.of.the.person.com
then type /mode #channelname +b *!*@address.of.the.person.com
Thats about all you can really do. IP address bans won't work in channels unless his IP address does not reverse DNS and the network you are does not have address cloaking.
Try contacting an IRCop about it, they have more abilities to remove such people from the network entirely.