How to disallow user to register with a specific word in the username?

Question

Following code should work for you, but I can not guarantee that because your form is so heavily (and badly) customized. I have tested it on not customized registration form and it worked just fine.

function cyhorge_check_fields( $errors, $sanitized_user_login, $user_email ) {

    if ( preg_match('/(CyhorgeVH)/', $sanitized_user_login ) ) {
        $errors->add( 'username_error', __( '<strong>ERROR</strong>: User contains CyhorgeVH string.', 'my_textdomain' ) );
    }

    return $errors;
}

add_filter( 'registration_errors', 'cyhorge_check_fields', 10, 3 );

SOURCE

EDIT: Another possible solution:

function prevent_cyhorge_user( $user_login, $user_email, $errors ) {

    if ( strpos( $user_login, 'CyhorgeVH' ) ) {
        $errors->add( 'username_error', __( '<strong>ERROR</strong>: User contains CyhorgeVH string.', 'my_textdomain' ) );
    }

}

add_action( 'register_post', 'prevent_cyhorge_user', 10, 3 );

Notice that the second function is using strpos instead of preg_match which probably a bit faster, but preg_match provides more flexibility if needed.

Both should work from either theme’s functions.php or as a plugin.