Set up username and e-mail address access rules for new and existing accounts (currently logged in accounts will not be logged out). If a username or e-mail address for an account matches any deny rule, but not an allow rule, then the account will not be allowed to be created or to log in. A host rule is effective for every page view, not just registrations.
Select Add rule to add a rule. You can choose between an access/deny rule and apply it to username, e-mail or host mask and use wildcards. Using the Check rule you can test whether it applies.