The only way you can stop spam with money
The corollary of the funding capture issue is that the only way for an anti-spam scheme involving money to work is for the people receiving email to pay for it; only then are the economic interests kept aligned. Naturally this is not very popular with people getting spammed, which helps to explain the enduring popularity of proposed schemes where someone else pays.
Unfortunately, this creates a perverse incentive that's familiar from the anti-virus world: if spam goes away, so do the anti-spam organizations. Their continued existence is in practice based on the very thing they are theoretically supposed to make ineffectual, and it helps them when this thing is seen as a big menace.
(Thus I am not sure I trust various anti-spam vendors' numbers for spam volume. Regrettably, their scary figures do roughly agree with my own back of the envelope numbers.)
The good news is that I'm pretty convinced that the perverse incentive isn't very big so far, which is also bad news because it means that the overall spam problem really is that bad. And I don't believe it's going to get better any time soon, especially since the future of spam is likely to be hard to stop.
(Arguably the virus writers used to be encouraged by the publicity that anti-virus companies gave them, but spammers have been driven by money right from the start.)