With more and more newcomers arriving on the Net, the law of averages suggests that a growing number will be the sort you wish had never been let near a keyboard. The kill file is basically a filter that throws away messages according to criteria that you set up.
Commonly it is applied in a particular newsgroup to a particular person or topic. Messages matching the criteria don't appear on your screen. So while everyone else might have to read their daft replies, you can serenely get on with reading only the incisive, witty debate going on. More advanced newsreaders can exclude a person or subject by newsgroup, region, or globally. The longer you've been on the Net, the more people and topics your kill file is likely to contain.
Kill files first appeared in 1984 - by which time Usenet (as newsgroups are collectively known) was just five years old. It was the invention of Larry Wall, who in his spare time wrote a program called readnews(1) including the facility. Clearly he had already had enough by then of some of the daft stuff posted to newsgroups.
Kill files do not have etiquette as such, since they are (almost by definition) the complete opposite of etiquette: you're blanking another Net user out entirely. Occasionally somebody will in effect announce that they are "killfiling" somebody else by replying to one of the annoying person's posts with the word "plonk". This is meant to be the sound of someone taking the long fall to the bottom of the depths of soundlessness.
The worst situation to be in is having newsreading software - such as a Web browser - that doesn't have a kill file, as it makes some of the busier newsgroups impossible to work through. Since newsreaders are plentiful, and free or shareware, it's worth looking around. For advice, try the newsgroups news.software.readers and alt.usenet.offline-readers.Reuse content