It rarely does, which is why these aholes get to carry on doing it all over the net.
All it takes is a redirect to a page that says:
We apologise for the inconvenience, but access to this site is denied to all users from your ISP.
We have done this as they refuse to enforce a reasonable standard of behavior from their users, closing the accounts of any abusive users.
Please contact your ISP to inform them of your dissatisfaction at their standards of service.
I could block a whole ISP from accessing my webserver in a matter of minutes like this, and while it doesn't have enough users to get a result, I've seen plenty of accounts of this working for bigger sites.
He's not just impersonating people, he's spreading trojans that will probably be used to collect passwords and commit fraud. That kind of behavior does deserve an ISP account closure, and if websites won't use their influence to encourage foreign ISPs to stop it who will?