Blacklist of domains for registration form

Hi Roman Vader,

Thank you for reaching out.

You can use the form filters to validate any user field before creating a new user. For example, the user name abc123 will not be created/updated.

add_filter( 'rwmb_profile_validate', function( $is_valid, $config ) {
    if( $_POST['user_login'] == 'abc123' ) {
        $is_valid = false;
    }
    return $is_valid;
}, 99, 2 );

Get more details on this documentation https://docs.metabox.io/extensions/mb-user-profile/#form-fitlers.