A ban is the removal of a player from a clan. This will often include them not being welcome in the Clan Chat, not being welcome at clan events and not being welcome on the clan thread. There are generally two types of ban: temporary and permanent.
If a player is temporarily banned, they will not be welcome in the clan for a period of time. They will usually be added to the Ignore list of the Clan Chat owner to stop them from entering the Clan Chat. They will also not be welcome at clan events or on the clan thread. However, they will usually remain on the list of members (if there is one), and will be allowed back into the clan after a period of time. Some clans will let the user back in on their old Clan Chat rank; others will make them start from Recruit.
If a players is permanently banned from a clan, they must have committed a serious offence (either repeatedly breaking rules, or breaking one big rule, such as backstabbing or making very offensive comments). When a player is permanently banned, the same will happen to one who is temporarily banned, but they will not be allowed back in. They will also have their name removed from any member lists.