To ban users from your YaBB forum, please follow these steps:

1. Log into the YaBB administrator area at http://www.yourdomainname.com/cgi-bin/yabb/YaBB.cgi?action=login (where yourdomainnam.com is your domain name)
2. Select Admin link
3. Select Ban Members link from the members control box.
4. From the onscreen prompts, ban a specific user or an individual or
range of IP addresses or hostnames. These methods prevent a user from even reaching the index page of your board. To prevent a user from registering under a different username you
can also specify a banned email address. Please note that banning an email address alone will not prevent that user from being able to log on or post to your board. You should use one of the first two methods to achieve this.

This document is FAQ559.