You can also set up individual privileges to users. To do that, please follow the Permissions link in the User Admin menu. Type in the username you would like the change the privileges for and click the Look up User button.
The next screen will allow you to change the access level of the user, as well as edit his moderator privileges.
Clicking the Submit button will apply the changes immediately.
Through the Ban Control link you can deny access to users by different criteria: be it username, e-mail address or IP address.
It is important to remember that banning an IP address will disallow all users, who use that IP, to view your forum.