So says, Bruce Schneier in a Forbes.com article here: ("Why Spam Won't Go Away").  He packs several different points into a thousand word piece.  The crux:

Spam is such a common tactic not because it's particularly effective--the response rates for spam are very low--but because it's ridiculously cheap. Typically, spammers charge less than a hundredth of a cent per recipient. And that number is what spamming houses charge their customers to deliver spam; if you're a clever hacker, you can build your own spam network for much less money.

If it's worth $10 for you to successfully influence one person--to buy your product, vote for your guy, whatever--then you only need a 1-in-100,000 success rate. You can market really marginal products with spam.

However, this cost/benefit calculation is missing a component: the cost to the recipient. Spam costs corporations millions in Internet capacity, clogs up infrastructure, requires people and products to deal with it and wastes employees' time wading through whatever spam makes it into their inboxes.

There are also less tangible costs. Marketing messages annoy. The advertiser pays part of the cost of annoying people if they decide to boycott his product. But more of the cost is paid by the receiver: the beauty of the landscape is ruined by the billboard, dinner is disrupted by a telemarketer, spam makes e-mail a more annoying task and so on.

This is why spam is such a hard problem to solve. For each e-mail, the spammer pays a cost and receives a benefit. But there is an additional cost paid by the e-mail recipient. Because so much spam is unwanted, that additional cost is huge--and it's a cost that the spammer never sees. If spammers could be made to bear the total cost of spam, then its level would be more along the lines of what society would find acceptable.

The best solutions raise the cost of sending spam. Spam filters raise the cost by increasing the amount of spam that someone needs to send before someone will read it. If 99% of all spam is filtered into trash, then sending spam becomes 100 times more expensive.

There's probably too much to unpack here, but the piece mulls over several solutions, ranging from private sector/technology-based solutions to legal solutions (imposing penalties for spam – as the previous post indicates, this is one approach being taken in the U.S.).

My take:  the technological solution is not working at present.  Nor is there any indication that it ever will, mostly due to adoption issues.  Nor is the existing U.S. legal regime working.  The article hits on this last point, noting that "[legal regulation] only works ehen the spammer is within the reach of the law; it's less effective against criminals who are already committing fraud and using spam merely as a mechanism."  In fact I would submit that the existing legal rules in the U.S. (i.e., CAN-SPAM) make it more competitive for out of jurisdiction spammers and merely tax/penalize the ones who can be found – i.e., those who essentially comply with the law.  (I'm thinking of the distinction between the Lithuanian spammer who surreptitiously uses servers in Korea vs. a U.S.-based company who sends email on behalf of its customers and identifies itself in the process.  Guess which one is more likely to be sued under CAN-SPAM?)  Any legal solution has to involve a coordinated international effort.  Anything less may well exacerbate the problem. 

To be sure, this is a complicated topic, and the article offers some food for thought.  On the personal level, [via Life Hacker] this counsel seems sage (Smashing Magazine: "Preventing Spam: Bulletproof Solutions"):

The most important rule to avoid spam is never mention it somewhere in the Web. So what I’ve suggested to do is to create two e-mail-accounts - the one for business contacts, which will be used only for communication with partners and serious clients and the second one, which will be decoded and published on the Web for any other purposes.