Have you considered not connecting your critical infrastructure directly to the Internet. The fact that the 'Cyber attackers' can even see your computers shows extreme complacency by whoever is in charge of your 'computers'.
For all we know, their network wasn't attached to the Internet and that there was an air gap between it and the outside. Problem is, it isn't terribly difficult to insert your own back door. In many cases, you just need a wireless adapter and the proper software. Even if they're not running an IP network, you can encapsulate their traffic and send it through your eavesdropping device.