|Someone wrote in mjg59,|
My sympathies for your lack of traction with law enforcement; I wish we could get as much effort put into tracking down anonymous threats as we currently have for anonymous copyright infringers. Not that that will solve the problem, but it would certainly help to stop all the harassers too stupid to use Tor, as well as the ones smart enough to use Tor but afraid of what they see happen to the ones that aren't.
It bothers me immensely to hear that despite your rapport with the FBI cybercrime office you still couldn't get these threats tracked down. That's really discouraging. All I can say is, if I had the resources to throw at you to help you solve this problem the way people with money manage to get such problems solved, I'd do it.
As for ISPs, most of their terms of service *explicitly* forbid harassment and similar behavior, so it saddens me to hear that that apparently remains just empty talk. Seems like a disconnect-on-first-offense kind of issue, to me.