ll_kill_frontend_rewrite_rules_update

add_filter( 'pre_update_option', 'll_kill_frontend_rewrite_rules_update', 10, 3 );
function ll_kill_frontend_rewrite_rules_update( $value, $option, $old_value ) {

	// We're only concerned with the rewrite rules option.
	if ( 'rewrite_rules' !== $option ) {
		return $value;
	}

	// We want to concentrate on frontend requests only.
	if ( ! is_admin() && ! is_user_logged_in() ) {
		// Returning the old value will effectively stop the update.
		// See https://core.trac.wordpress.org/browser/tags/4.8.3/src/wp-includes/option.php#L307
		return $old_value;
	}

	return $value;
}