Which is what most spammers do, hence my statement Spam is almost never traceable, it usually comes from a bunk address Stands...
Actually it's pretty easy to trace, the problem is most users don't know where to look. The original IP is almost always in the header, so just run a WHOIS on it and forward to the owner of the address. That's how I shut down the guy who hacked Free Dominion and sent email threats to the membership.
1. Most spammers don't bother because they're not in the US and could care less if the spambots they're using are blacklisted.
2. It's easy to check the mailing headers and verify the chains. If one of the hops is an open proxy, you know that anything prior to that is bogus, and there are a number of locations which will give you a free ID and a web interface which will automatically do this for you.
Believe what you like, but you're wrong.