Usage: UNBAN <#channel> [<*>]
If issued without the '*' parameter, it will remove all bans that would affect you. With the '*' parameter, all bans are lifted.
You need high enough access to the channel to use this command.
/unban #mychannel *
This is a convenient and fast way to clear the entire ban list of a channel, but it can also be used if you find yourself banned from a channel you have access in, as you can remove the bans from outside and then join.
This replaces the service command /CS UNBAN.