[Dave]

I've seen this before, they simply proxify all requests to your server and imitate the request as a GoogleBot so it's hard to block it.

Add the following to your headinclude template and make sure to modify the second line.
For example if your domain is vbulletin.org, split it in half and change it to "vbull" + "etin.org". This must be done so it's harder for them to replace all instances of your domain with their own.

<script>
   if(window.location.hostname.indexOf("your" + "site.com") === -1){
      window.location = 'https://www.go' + 'ogle.com';
   }
</script>

What this script does is it checks if the current URL contains the domain you supplied, if not, it will redirect the user to Google.
Essentially this will work against their intentions (injecting ads on the cloned forum) since all users that visit the cloned site of your forum are redirected to Google.