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Restrict the usernames that new users may use when registering for your site.

If open registration is enabled for your site (via Settings → General → Membership (“Anyone can register”)), WordPress allows visitors to register for an account on your blog. By default, any username they choose is allowed so long as it isn’t an already existing account and it doesn’t include invalid (i.e. non-alphanumeric) characters.

Possible reasons for wanting to restrict certain usernames:
* Prevent usernames that contain foul, offensive, or otherwise undesired words
* Prevent squatting on usernames that you may want to use in the future (but don’t want to actually create the account for just yet) (essentially placing a hold on the username)
* Prevent official-sounding usernames from being used (i.e. help, support, pr, info)
* Prevent official username syntax from being used (i.e. if all of your admins use a prefix to identify themselves, you don’t want a visitor to use that prefix)

When attempting to register with a restricted username, the visitor will be given an error notice that says:
ERROR: This username is invalid. Please enter a valid username.

NOTE: This plugin does not put any restrictions on usernames that the admin chooses for users when creating user accounts from within the WordPress admin. This only restricts the names that users choose themselves when registering for your site.

Find out more at the plugin’s WordPress Plugin Repository page.

1. Unzip ‘restrict-usernames.zip’ inside the ‘/wp-content/plugins/’ directory for your site (or install via the built-in WordPress plugin installer)
2. Activate the plugin through the ‘Plugins’ admin menu in WordPress
3. Go to the Users -> Name Restrictions admin settings page. Specify username restrictions.