Based on the existing information, it seems that the person in question has left the group and is not listed in the members tab. However, it is possible that the person in question has sent messages to the group and can be searched for in the messages section. As such, it is likely that the group has a watch account in the group, and uses another account to send direct messages. This allows the group to monitor activity and take action as necessary. In order to ban the person in question, the group should search for any messages that the person in question has sent, and then take the appropriate action to ban them from the group.
