This article suggests fighting spam by blocking out the sites that host spam related products:
This is an intersting approach to making spam less profitable and protect people on networks where these web sites are blocked, though it may not initially affect the amoung of mail in people's in boxes. The spam will still come - you just won't be able to get to the web site (which is good).
I think penalizing the web hosts that knowingly host these web sites for spammers when it is clear what they are doing is a better approach. Drive up the cost of running these businesses and send the people who support them to jail as accomplices.
Being a hosting company I know this is tricky. I don't want to go to jail because one of my customers sent spam - so this would have to be done knowingly. There would have to be proof of the actions and that may prove difficult when the hosting company says "we didn't know".
However requiring hosting companys that have had 2-3 spam incidents to perform certain types of audits like monitoring outbound mail traffic levels and such might help. If they see an exhorbitant amount of mail coming from a particular customer the should be able to determine if the actions are suspect. Typically you can tell a crappy, spammy web site or email when you see it. You can also find out if that company has a double opt in policy and a clear way to get off the list.
Another concept would be to require spammers to include footers that send complaints back to the top level hosting providers or networks. Each spam message would need to have a clear and easy to read abuse email address that goes to the hosting provider that way they cannot say "we didn't know"...
Obviously there are a lot of web hosting companies and server owners whose systems and networks abused without their knowledge, but some of these people are catering to the bad guys. Those people should be penalized along with the people they support.