Server administrators can ban players from their server. There are several methods of banning, including banning ranges of addresses. Ban durations can vary from minutes to "forever".
Caught in a ban
If a player is caught in a ban, the server will refuse to allow the player to join. A message should appear at the very bottom of the chat box that says why the ban is in place, and who created the ban. This ban may not necessarily be targeting the player that is joining. Many times an admin or cop will have to enter a range ban in order to keep a trouble player off the server. This can have the side-effect of banning innocent players as well.
Sometimes the player will not see a message. This is usually the case when caught in a host ban (a ban on the hostname instead of the IP). The world will show up, but the player will be unable to spawn and the connection will drop shortly after. For this, the player will have to contact the server owner or an administrator to find out where the issue lies.
Contacting an administrator
A ban may be lifted or worked around by contacting an administrator. Generally it is best to contact the administrator or cop that issued the ban. This can be found at the end of the ban message when joining.
One way to do this is to send a private message to the user on the BZFlag forum. The admin or cop might also be online somewhere else.
Do NOT post a forum thread asking to be unbanned. This is a private matter and it should be handled in private.
Many servers use a "whitelist" group for players mistakenly caught in bans. This group would be assigned a permission that allows them to join through any existing bans. This allows the server to keep the trouble players out while also allowing others caught in the ban to still play. This would require that the innocent players play under a registered account and that the account is added to the correct whitelist group.
If a player causes a change on their network in order to evade a ban, they can expect their existing ban to be updated to include a range of addresses and a much longer ban duration. It is better to accept the ban and/or contact the administrator to find out how long the current ban will last.