Unban
To unban a user type
/mode #yourChannel -b <hostmask>
(you need to be halfop or higher).
If you don't know the hostmask type
/mode #yourChannel b
or use the mibbit default-alias:
/ban<space><enter>
If the banlist is not empty you will get a list with unban buttons (using a mibbit client).
If you are banned from your own #channel or if you are on the aop list there type
/cs unban #channel