The solutions may well be found there but will be unimplementable without much needed support from the operators - particularly the major backbones - who currently turn a blind eye to protect their revenue.
Bingo. There's the crux of the problem. It needs to be elaborated on and emphasized, because most engineers have a blind spot about the business aspects of their industry (no matter what that industry is). There's a lot of wailing and gnashing of teeth, a lot of soul searching, a lot of angst here. All for naught. Many big network operators are selling bigger and bigger pipes to everyone so they can keep up with more and more spam. They make money on the increased traffic, even as they have these solemn terms and conditions in place about how they won't tolerate spam. The big network operators don't need to allow spammers to connect directly to their backbones. They make money by selling transit to other networks who sell transit to still other networks who then allow spammers to connect. Network operators are such a naive bunch of engineers. There's lots of money to be made just in transit for spam, and quite often the people who sign the paychecks for the engineers who post to this list are the very people who benefit. They understand this, why don't you? Every network operator should first try to get their own company to get serious about stopping spam. Top management has to be willing to do what it takes. E.g. de-peer, stop selling transit, etc. Until that happens the spam problem will keep getting worse. And if top management isn't interested, or won't agree to do anything meaningful, ask yourself why. And keep that in mind the next time you get paid.