An easy way is for each ISP only allow outgoing mail via their main mail server, all others have to smart relay through it...
ISP's block Port 25 on it's border router except to it's mailserver so only mail leaving an ISP's
own network is filtered through it's Mail server.
This would stop all the SPAM in a matter of days....
My ISP is set up that way.... and there are at least 7 mail servers on my ISP's system

My Mailservers all relay outgoing e-mail via my ISP's main mail server
If a spammer had to do this, it would be real easy to track and
the ISP would see it happening.....
Most spammers use cable or ADSL circuits to do their dirty work....
If the smart relay took off... and ISP's do not want their
regular customers mail blocked... Guess what would happen....

I do like to use a free spam blocking software from www.spampal.us and it
does work well....

SPAM... Breakfast of Jerks!