do-not-sync-with-selected-sites.php

<?php
/**
 * change 3,4,5 with your own site IDs where you don't want to sync stock. Each ID
 * should be seperated by a comma. 
 */ 
$site_ids = array( 3, 4, 5);

/**
 * Do not edit anything below.
 **/
$all_sites = get_sites();

foreach( $all_sites as $site ) {
  if ( in_array( $site->blog_id, $all_sites) {
    add_filter('WOO_MSTORE_SYNC/settings/is_stock_sync', '__return_false');
  }
}