Google wouldn't know but the ISP would know who was behind a particular IP at a specific time. They are the ones who should police their network when there are abuses.
The original proposal was that google delivers the ads. So google would have to contact my ISP who would then have to return whether or not I was using any of the given "spammy" IPs at the time that they were spammy - or my ISP would have to deliver the banner.
No thanks.
Other people share a network behind a NATed IP which is also a problem. They'd all receive a banner, check their computer and a test would come up negative.