While surfing on the net I found Spam D Spammer, a tool to fight spammers and hit them where it hurts them most: $.
The setup seems simple, you install the software, receive spam with the URL of their site and start downloading their site. When a lot of people do the same the bandwith usage of the spammers site will start growing until the costs are too high for the spammer.
Seems simple and effective but just a few thoughts. When spammers use URLs in their spam they often change the URL to subdomains, other aliasses and so on. In general their URLs change every 24h or less. So when you want to attack the spammers web sites you are spending quite some time to track the spammers sites. At MX Lab (www.mxlab.be) I see about +150 new URLs pass every day. If the authors decide which sites are being top priority targets and the software is feeded with this information you have less work to do.
Besides the increased bandwith that the spammer will have you are also using your own bandwith or datatraffic from your ISP. At the end of the day you will be attacking several sites so if you have no xDSL or broadband you will certainly make your internet connection crawl. And from what I have read you can control the used bandwith from this software to increase your own bandwith again. It says enough.