The Washington Post's "Security Fix" blog has managed to shut down McColo, a US-based web hosting provider which facilitated more than 75 percent of global spam:

A U.S. based Web hosting firm that security experts say was responsible for facilitating more than 75 percent of the junk e-mail blasted out each day globally has been knocked offline following reports from Security Fix on evidence gathered about suspicious activity emanating from the network.

For the past four months, Security Fix has been gathering data from the security industry about McColo Corp., a San Jose, Calif., based Web hosting service whose client list experts say includes some of the most disreputable cyber-criminal gangs in business today.

Major Source of Online Scams and Spams Knocked Offline [SecurityFix]
Host of Internet Spam Groups is Cut Off [WashingtonPost]