In the Command Reference wiki entry, /ignore is specified. There is no counterpart to remove someone from ignore.
Witnessed behaviour: I used /ignore [username] and the user was successfully added to ignore. Then I wished to unignore the user, but did not know what to type. I clicked the 'ignore' on the user's tray->right click on user name menu, but it wasn't checked in the first place. Typing /unignore had no effect.
Workaround: I could type /ignore, pull up the list, highlight the user's name and remove them from the list to unignore them. This isn't incredibly obvious, though.
Other workaround: Restarting the Colloquy program also cleared the ignore list.
Oddball behaviour: I opened the console for said server, and I could see the user's messages in there, so I know it wasn't server side.
ex: [code]TheUser? PRIVMSG #thechannel typing stuff here/code
Expected behaviour: If I've /ignored someone and it was client-side, then I would expect to a) see the ignore check marked in their user info, and b) expect that an unignore command would work.