Great question! Based on the forum discussions here, it's important to understand that username validation in wpForo is actually handled by WordPress core functions rather than wpForo itself. Since WordPress natively allows spaces in usernames by default, you'll need to implement a custom solution at the WordPress level to restrict this.

Here's a step-by-step approach to prevent spaces in usernames:

1. Use the sanitize_user filter: WordPress provides a filter called sanitize_user that you can hook into to modify username sanitization. Add this code to your theme's functions.php file or create a custom plugin:

   <?php
   add_filter( 'sanitize_user', 'prevent_spaces_in_username', 10, 3 );
   
   function prevent_spaces_in_username( $username, $raw_username, $strict ) {
       if ( strpos( $username, ' ' ) !== false ) {
           return '';
       }
       return $username;
   }
   ?>

2. Add client-side validation: While server-side validation is essential, also add JavaScript validation to the registration form to provide immediate feedback to users. This improves user experience by catching the issue before form submission.
3. Use the registration_errors filter: For more robust validation, you can also hook into registration_errors to display a custom error message if someone tries to register with spaces:

   <?php
   add_filter( 'registration_errors', 'check_username_spaces', 10, 3 );
   
   function check_username_spaces( $errors, $sanitized_user_login, $user_email ) {
       if ( isset( $_POST ) && strpos( $_POST, ' ' ) !== false ) {
           $errors->add( 'username_spaces', __( 'Usernames cannot contain spaces.', 'your-text-domain' ) );
       }
       return $errors;
   }
   ?>

4. Test thoroughly: After implementing these filters, test the registration process to ensure spaces are properly rejected and users receive clear feedback about the requirement.

Since these are WordPress core functions, this approach will work with wpForo's registration form as well. Have you already tried implementing custom filters in your WordPress installation, or would you prefer a plugin-based solution instead?