Comment Some already do (Score 1) 486
I work at a decent sized regional ISP. If a customer is disrupting the network with blatantly viral traffic (like tens of thousands of simultaneous SMTP connections) we shut them off and have tech support walk them through disinfecting their PC. The exception is if they also have VOIP through us since we don't want to be in the position of having cut off someone's only link to 911.
The network engineers don't sit around all day looking for infected boxes, but if performance issues are traced to an infected customer they definitely get cut off.