Whereby a bot (on an oft-compromised machine) is used to spew messages through an open NNTP server. These bots are designed to avoid detection by other bots and make UseNet unpleasant for its users (and administrators). 

A Usenet death penalty is generally the only way of stopping (read: stalling) such an attack. 

Standard google search:

http://www.google.com/search?q=usenet+death+penalty

Google groups search:

http://groups.google.com/groups?q=usenet+death+penalty&meta=site%3Dgroups

Google search for hipcrime:

http://www.google.com/search?q=hipcrime

As a further note, current floods on usenet -cannot- be filtered by keywords or content. Bots have been written to create random posts with structured syntax in a manner impossible to spot with an antispam bot. -- DominicBurns