[rudedog]

I think he means they are resetting their connection ( ipconfig /release and /renew )

The only way to do this is with autokick

It's much easier for you to ban them every time they come back then it is for them to release and renew.

Like you said by banning a complete subnet. There is a possibility that your banning a good player.

Also with autokick you can ban a name as well. this way if they come in with that name or IP they will be banned.

Good Luck this is a tricky situation.