I'd suggest making some critical function for your program be a webservice on a server you control. Then you can prevent access to the service using a firewall. That way, if the exe escapes your network, it can't do anything useful because a critical bit of functionality is left out. Just make sure that the function isn't trivial to reimplement, or someone may do so.

...roboticus

When your only tool is a hammer, all problems look like your thumb.