We all know the story on spam: spammers are beyond the reach of the law (jurisdictionally or just because they can hide), so punishing them can't make a dent. Spamming is already understood to be evil, so anyone doing it is beyond being embarassed into obeying the social will. Technological measures help, but imperfectly. Sending spam is cheap, and always will be. Accordingly, ads that fail completely on the mainstream consumer clogs everyone's inbox for so long as some weirdos keep buying.
It seems to me that the buyers are the weak link here. Their participation is obviously vital to the success of the enterprise. Unlike the customers of traditional crime -- people who buy stolen goods, drugs, the services of hookers -- the idiots taken in by spam strike me as particularly susceptible to some forced re-education in societal standards.
Here's what I propose: the government should fine, ban from the net, or jail any "victim" of spam. "Spam" would cover only messages illegal under standards similar to those in existing laws -- e.g., messages which were required to have, but did not have, a particular tag indicating commercial content. "Victim" would be anyone who purchases a good or service, makes an investment, makes a donation, or otherwise benefits the illegal sender, as a result of the illegal message, and who knew or should have known that the message was illegal or improper.
Stay with me here. The reason for laws against child pornography is to protect children from its production, but the government punishes people who purchase, sell, possess, or distribute it, as well, to fight the profit motive of producers. It even distributes some itself, to root out the potential buyers who would otherwise support the production. Likewise, the FBI (or Postal Inspector or whatever) should get into the spam business, buying commercially available lists of addresses, sending illegal messages with properties that make them easy for the educated to filter, to seek and destroy the spam consumers who make that blight possible.
Of course, a determined recipient could work around this stuff. But he would find it a lot easier to get the spammers to cut out the fraud, mislabelling, false return addresses, etc., which would not hurt the recipients at all (unlike, e.g., bans on drugs, which drug purchasers do indeed sincerely want).
I see two weaknesses to this scheme. First, people who are taken in by spam, especially 419 scams and the like, may be so senile or stupid that the law can't help them. Maybe so, but I urge anyone who thinks this to consider whether such people should really be entitled to receive and send email, given the harm they're causing for all of us.
Second, there would be tough borderline cases. We wouldn't want GM to be able to send pro-Ford spam and thereby make buying Fords illegal; on the other hand, Ford shouldn't be able to send spam just by pretending it was sent by an independent party. I am confident, though, that a minimally inclusive law would offer little potential for malicious use, but would still snare the frequent spam purchaser.