A cancelbot is a program or bot (robot) that sends a message to one or more Usenet newsgroups to cancel (remove from posting) a certain type of message. It searches for messages matching a certain pattern, whether it be a duplicate message or offensive material, and sends out cancels for them. When a message has been canceled, its status is changed to "cancel," and the Usenet servers will no longer post them.
Some Usenet users consider cancelbots a form of censorship. Many Usenet newsgroup administrators, however, believe that they have a right to cancel material they consider to be offensive or unwanted, such as spam (bulk mail). Anyone with authority to send a cancelbot has to be careful to make sure that they don't cancel more messages than they intended to.