The changing the username is a good idea however people that are 'threatened' or want to start a fresh is a good point, but those people can report the user that 'threatend' them therefore the system would identify them by the ip address. The abusers could start a fresh awell which is a catch because they could start abusing again under that new username.
Alienuk, sometimes different users work from the same computer; sometimes users work from internet cafes; I don't think there's really any foolproof method (otherwise certain users would be banned more often than they are already).