(orig. Usenet usage)
A list of undesirables, either by name, email address, or regular expression, whose messages will be automatically removed before reading. Generally the place of residence of trolls, lamers, and flamers. Common residents are Grubor, Sternlight, and Windigo the Feral

"Excellent. What is a killfile?"

"Uh. It's a list of usernames/topics/news items etc that you wish the newsreader to automatically skip so you don't have to wade through rubbish"

"Uh No. Remember I said pertaining to Operations. A killfile is in fact a file with a list of names of people you are going to have killed."

"Oh. Of course."

- The Bastard System Manager From Hell 2
Simon Travaglia

a killfile is the ultimate free speech device: it does not hurt other people's right to speak, but it empowers the reader, since it gives hir a right not to listen.
Now, if we only had killfiles in RL. Wait, we have them: but they are normally compiled inside the brains of secretaries.