Plonk is a Usenet jargon term for adding a particular poster to one's kill file such that the poster's future postings are completely ignored. It was first used in 1989, and by 1994 was a commonly used term on Usenet regarding kill file additions.
The word is an example of onomatopoeia, intended to humorously represent the supposed sound of the user hitting the bottom of the kill file (imagining perhaps the kill file as a bit bucket). It is also sometimes given as an acronym standing for Please Log Off, Net Kook, though this is a backronym. Other used expressions are "put lamer on killfile" and Please Leave Our Newsgroup: Killfile!
It is also used as a verb: "I plonked that idiot". As a public repudiation of the plonked poster, it is appended to the end of one's reply (or may constitute the entire reply). A user might also simply reply to the impugned post with the word "Plonk".
The term's usage later expanded to include the use of e-mail filters that delete incoming messages that meet certain filter criteria set by the receiving user, so block messages from annoying senders. It has also often been figuratively used on BBSes, webboards, blogs, IRC, and wikis (which usually do not actually have filters), and is occasionally used in reference to blocking a user on an IM protocol or a social media site.
First known use
>>Please refrain from posting to talk.bizarre until such time as you >>cease to be an asshole and become at least one of: bizarre, creative, >>or entertaining. You are welcome to dump your rotting ordure in rec.humor >>or some similar group where your fellow mental defectives congregate. >Make me. *plonk*