One relatively well-known example of a system that increases the cost of hosting a spam-advertised web site is Spam Vampire. Spam Vampire is a web page containing a collection of links to images that were advertised in spam e-mail. A person who is tired of getting spam loads the web page in his browser, and the browser reloads the images indefinitely. As the images are loaded and reloaded, the web traffic is increased at the web site that was advertised, making it more expensive to host the site (the cost of web site hosting increases with the activity on the web site). The goal is to make the cost of the increased traffic approach the profit derived from sending the spam so that there will no longer be a net profit, and the spam will stop.
This strategy would be very effective if enough people participated, and it shouldn't take very many people either. The originator of Spam Vampire reported a significant decrease in his volume of spam, and a company that went after spammers (Blue Security) was very successful, as demonstrated by the subsequent vicious counterattack. My educated guess of the number of people using DSL or cable that it would take to bring most spam to an end would be in the thousands. Considering how many people use the internet worldwide (> one billion?), this means each person who does something is very powerful.
Since this method is powerful, it can also be dangerous. Here are some caveats:
If the amount of traffic at the spam advertised web site is overwelming, the webmaster can redirect traffic at an innocent web site by changing the IP address associated with the web site's domain.
Costs increase not only for the spam advertised web site, but for all servers between the web site and the computer downloading content. It may only be the end user that has "unlimited usage". This could lead to complaints from one's internet service provider.
Reduces spam for everyone
May impact non-spam-related web sites