|The only way you are going to do that is to put a cookie on the users machine when they log on that picks up machine IP address and sends it along with public IP address whenever they send anything to the games server.|
You are then going have to have some software on the game server that monitors incoming packets and checks if there are any public IPs with different machine IPs.
Needs some pretty neat programming but it can be done.
BTW: It is not a good idea to put your email address on a public forum. It is an open invitation to spammers. Besides. the idea is that you answer a question, it gets answered and then when somebody else comes along they also can see the answer.