A
thing that lurks in the
Usenet. Destroyer of
spam, eater of
junk
and
friend of
users.
CancelMoose (actually, these days, a program called AutoMoose) doesn't send cancel messages to the Usenet. Instead, it sends "notices" to alt.nocem.misc. These notices (that can also be generated by spam warriors) will then be read by NoCeM or similiar program that will hide the offending articles from the user.
See also: http://www.cm.org/