Here is a snippet that enables you to restrict certain usernames from being created by new users registering to your WordPress website.

Instructions
Add this code your theme function.php file.
*Remember to replace content of the “restricted array” with the usernames you want to restrict.

<?php
   function wpsnippet_validate_username($valid, $username) {
   $restricted = array('profile', 'directory', 'domain', 'download', 'downloads', 'edit', 'editor', 'email', 'ecommerce', 'forum', 'forums', 'favorite', 'feedback', 'follow', 'files', 'gadget', 'gadgets', 'games', 'guest', 'group', 'groups', 'homepage', 'hosting', 'hostname', 'httpd', 'https', 'information', 'image', 'images', 'index', 'invite', 'intranet', 'indice', 'iphone', 'javascript', 'knowledgebase', 'lists','websites', 'webmaster', 'workshop', 'yourname', 'yourusername', 'yoursite', 'yourdomain');
   $pages = get_pages();
   foreach ($pages as $page) {
      $restricted[] = $page->post_name;
   }
   if(!$valid || is_user_logged_in() && current_user_can('create_users') ) return $valid;
   $username = strtolower($username);
   if ($valid && strpos( $username, ' ' ) !== false) $valid=false;
   if ($valid && in_array( $username, $restricted )) $valid=false;
   if ($valid && strlen($username) < 5) $valid=false;
   return $valid;
   }
   add_filter('validate_username', 'wpsnippet_validate_username', 10, 2);

   function wpsnippet_registration_errors($errors) {
      if ( isset( $errors->errors['invalid_username'] ) )
         $errors->errors['invalid_username'][0] = __( 'ERROR: Invalid username.', 'wpsnippet' );
      return $errors;
   }
   add_filter('registration_errors', 'wpsnippet_registration_errors');
?>

If you want to use a plugin to achieve the same functionality, you can use Scott Reilly’s Restrict Usernames WordPress plugin.

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