If you get kicked or banned from an IRC channel, you’ll find it impossible to get back in even if you change your username, nick, and IRC server.
The reason is that the ban applies to your identd, an identification which is provided not by your IRC client, but by the username supplied in your Network preferences panel. This is the same as your FTP/telnet username.
To get back into the channel after a kick/ban, jump offline, change this username, restart networking, and log back in. You’ll be all set.