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Generally speaking, nobody wants to be the cop that makes that call. Theoretically an ISP *might* be able to do that, but most are unwilling, and those of us that do actually play BOFH run the risk of losing customers to a sewerISP that doesn't.
Our experiences from the Dutch ISP market indicate otherwise, customers are more than happy to be informed they might have been infected by a virus/worm. Most customers are too afraid of loosing valuable documents due to a file-eating virus for example, or afraid of loosing connection to the internet entirely and appreciate it to get an opportunity to do some clean-up when placed in quarantaine vlan. They even will recommend you, and your reputation as ISP-with-clue will increase. To stay on-topic, this is one of the first steps to prevent hosts in your network attacking NATO and decrease the risk of being disconnected by them. Commercial products that might assist you: http://www.quarantainenet.nl/?language=en;page=product-qnet Michiel Klaver IT Professional