I was deleting some spam and reflecting on the whole issue... as I was telling someone a few weeks ago, the problem with spam will be stopped when it will become a serious issue. Right now, the problem is distributed: a few benefit and a lot have to pay for it. (Just like in politics [grin].) Worse, there is a socialism of sorts in the relationship between internet providers: the sender doesn't have to pay the carriers for the traffic it sends through them. This messes up the incentives completely.
A simple scheme like a hundredth of a cent per email would make it too expensive to send bulk emails ($100 per million emails) while not affecting regular users in any way; legitimate companies would have already a large enough internet subscription that any email charge would be drowned in the noise; finally, large, voluntary mailing lists like RISKS would need something on the order of maybe a few dollars a day - again, irrelevant when compared with the price for the actual equipment and internet service fee.
Furthermore, this scheme would create the proper incentives - the ISPs would penalize other ISPs who allow spam through them, the PC owners who don't secure them and let their PCs become spambots would suddenly have a financial interest to fix the problem, which would in turn put pressure on the OS makers (well, on Microsoft [grin]) to improve their software and so on.
Socialism works in small settings, like the family or small comunities, which is probably why it seems so natural to people. Free markets are much better at solving large-scale situations though. They can be thought of as real-life optimization machines - neural networks made of people, if you will.