Bearing in mind IRCds such as UnrealIRCd support a +e mode. That is a ban exemption. If a user, or any part of their full host matches any exemption, then any bans against them are therefore 'cancelled'.
You can view the exempt list by double clicking the channel window, you will see the ban list first, along with the title and some simple mode options. Hit the exempt button on the right hand side.
Alternativly, type /mode #channel-name-here +e, and then look in the status window.