The US firm's ultimatum follows a similar decision by Scandinavian ISP TeliaSonera, which sent warning letters to customers exploited by spammers.
Spammers were reported to have used customers' PCs as 'zombies', which acted as a spam distributor.
Comcast warned customers to keep their PCs clean after some of its IP addresses were discovered on anti-spam blacklists, which stopped some users sending emails.
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