Plonk is a Usenet jargon term for adding a particular poster to one's kill file so that that poster's future postings are completely ignored. It was first used in 1989, and by 1994 was a commonly used term on Usenet.
To publicly repudiate a poster, it is added to one's reply or is simply used as the entire, one-word reply. It is also used as a verb, as in: "I plonked that idiot".
Folk etymology sometimes gives the term's origin as an acronym of various phrases, although in truth they are backronyms. These backronyms include: Please Log Off, Net Kook; Put Lamer On Killfile and Please Leave Our Newsgroup: Killfile!
The term's usage later expanded to include blocking messages from annoying senders by using e-mail filters that delete incoming messages based on criteria set by the email recipient. Plonk has similarly been used on BBSes, online forums, blogs, IRC (Internet Relay Chat), and wikis (which usually do not have filters). It is occasionally used in reference to blocking a user on instant messaging (IM) 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*