Sustituir '@sibbnk' por la cadena que corresponda
add_filter( 'registration_errors', 'deny_sibbnk_usernames', 10, 3 );
function deny_sibbnk_usernames( $errors, $sanitized_user_login, $user_email ) {
if ( false !== strpos( $sanitized_user_login, '@sibbnk' ) ) {
$errors->add( 'username_not_allowed', __( 'Sorry, usernames containing "@sibbnk" are not allowed.', 'textdomain' ) );
}
return $errors;
}