depends on where they want to be unbanned from:
a channel: only an op ( the ppl with @ by their nicks) can unban someone.
a network: only an IRCop/admin can unban someone
in either case, a ban could be set to last a certain amount of time, and when that time is up, its removed. networks generally have an email addy listed or a forum on the network's website where you can contact someone to discuss the matter. with channels, you can try msging the op involved or the founder of the channel, or leave them a memo politely asking to discuss it.
