If you're feeling aggressive, join the crusade!
Get a traceroute utility (several are available from download.com). Now, trace that spam! From there, find the first legitemate company that is connected in the delivery of said evil-email. Visit their website and look for an anti-spam policy. If they have one, compose a polite email and send it including the spam-o-gram and the traceroute path that you got. You probably won't get a response due to the volume of mail these companies get, but know that you're email will not go ignored.
If that company doesn't have an anti-spam policy, move up the line (closer to your ISP) until you find one that does.
If everyone were to do this, we could probably greatly reduce the amount of spam out there.