lynt-install-block-mu.php

<?php
/**
 * Plugin Name: Lynt Install Blocker
 * Author:      Vladimir Smitka
 * Author URI:  https://lynt.cz/
 * License:     GNU General Public License v3 or later
 * License URI: http://www.gnu.org/licenses/gpl-3.0.html
*/

if ( defined( 'WP_INSTALLING' ) && WP_INSTALLING && !is_blog_installed() ) {
  if ($_SERVER['REMOTE_ADDR'] !== 'my.ip.add.ress' )  {
    wp_die('Installer blocked!');
  }
}