Maybe I should explain the idea in more detail.
Recall that the way Usenet works is that each site gets its new feeds from one or more other sites.
Imagine that I'm site A, connected to sites B and C. B is connected to D and E; C is connected to D, F & G. Whenever a user at a site submits an article, that site attests 'this message was sent via site X.' Whenever a site sends a message to another site that it received from somewhere else, it attests 'this message was sent via site X.' As part of the setup process, sites exchange signing keys (recall that Usenet peering was fairly static, something arranged by email).
So, any messages local to Site A don't (need) any attestation: they were generated locally, and read locally. Any messages submitted to Site B and sent to A are provably from Site B (and A would in fact reject any message claiming to be from Site B but without a signature). Likewise, any message submitted to Site E would be signed by E, then signed by B, then arrive at A. In the normal course of events, A doesn't care about Site E at all; he cares about the quality of his feed from B.
Say a spam arrives at A. He knows that it didn't come from C, because it's signed by B. So he returns it to B, stating, 'yo, police your users!' B can see his own signature on the spam, so he knows that he can trust the origination of the message from his point of view (whether it's his own user, or one from Sites D or E); in fact, B could automatically forward the spam notification to the originating site if it weren't his own.
Let's say that B is actually malicious, and has invented a hundred fake sites that he pretends to forward messages from. I don't care: any message I get from B is signed by B: if I receive enough spams from him, I can choose to just refuse articles from him.
> (X) Requires immediate total cooperation from everybody at once
Some solutions do require complete cooperation. Given that Usenet's effectively dead, is an example which kills Usenet and replaces it with Usenet-Prime all that different?
> (X) Anyone could anonymously destroy anyone else's career or business
Completely and totally wrong, since in the model I'm talking about reputation is local to my site, not shared.
> (X) Lack of centrally controlling authority for email
Nope: no centrally-controlling authority is needed: each site only cares about the relationships it has established, and doesn't care about downstream relationships ('police yourself!').
> (X) Dishonesty on the part of spammers themselves
Nope: dishonesty doesn't matter because each site only cares about the quality of the feed it receives from other sites. A dishonest site will have a poor-quality feed, and may be disciplined by being ignored.
> (X) Ideas similar to yours are easy to come up with, yet none have ever been shown practical
Seems quite practical to me. 'We've not bothered to try' doesn't count as a proof of impracticality.
> (X) Blacklists suck
Better than whitelists, no? At some point one has to refuse to do business with bad actors; that's a blacklist.
> (X) Countermeasures must work if phased in gradually
Why? As noted, Usenet is basically dead by now; how would killing it and recreating a new, more trustworthy one not have been better?
> (X) Why should we have to trust you and your servers?
I think maybe you thought that I was proposing some sort of centralised reputation system, which would indeed be crazy.