Cancel messages are a special type of news message that instruct all the servers it is posted on to cancel a particular message. These allow the original author of a message to remove it from all the news servers the cancel message can reach. Originally, the only persons allowed to cancel a message were the original authors. As Usenet evolved, the privilege was extended to the postmaster of the site that the cancel message was posted from. Some third party cancels are now possible, mainly by the moderator of a moderated news group.
Another acceptable use of cancel messages are automatic cancels directed towards Spam posts, since Spam is widely regarded as a major threat to Usenet.
There are obvious drawbacks to this method of controlling Spam. It's inconvenient. Somebody has to do the policeing. It's retroactive. It works after the fact, too late to prevent an inconvenience. Perhaps more importantly, it does nothing to relieve the overload of Usenet caused by Spam (see the message cancel faq).