NoCeM (No See 'Em, that is, the
spam) is a method of fighting
spam,
trolls and other unwanted
Usenet stuff.
Basically, it works like this: A NoCeM issuer sends "notices" (specially formatted, PGP signed lists of articles to be hidden) to alt.nocem.misc and probably cross-posting them to the relevant groups, if the message is not "global". The newsreader reads these notices and hides/displays articles according to the rules.
Anyone can issue the notices on different grounds (for example, to share a "killfile"); it's up to the reader to choose whether or not to heed to specific issuer's notices.
There is a program to apply the stuff to your own .newsrc, and stuff for inn to process NoCeM spam notices (from AutoMoose). Some newsreaders (like Gnus) also support NoCeM directly.
Personally, I usually issue NoCeM notices to keep rec.games.roguelike.nethack clean of warez d00dz.
For information on the NoCeM, see the FAQ at http://www.cm.org/faq.html
See also: CancelMoose