This looks like a good thing:

Comcast, the US cable giant, is threatening to disconnect customers whose infected PC are being used to relay spam messages.
Up to 30 per cent of spam is now spewing from hijacked “zombie” PCs. Spammers use Trojan horses and worms to take over PCs and use them as spam engines, a practice that can severely disrupt the operation of ISPs.

I’d like to see them extend this to include PCs infected with Blaster and other worms that continually spew out probes looking for machines to infect. There policy should provide a mechanism for these users to regain access once they have cleansed their PC.
There is some risk that Comcast will block some innocents as well. Remember, Comcast is the outfit with the Kafkaesque usage policy.