If you are trying to connect to a server and see a message telling you that you are banned from connecting to it, you would need to contact the owners or administrators of that network for help. They usually provide instructions in their ban message on what you need to do to contact them and/or to resolve the ban. For the Undernet network, you might be able to find help on their website
and, in this case, there is a post on their forums
that answers this question.