People would reach a point where further conversation makes no sense.
So, one would make a kill file entry, and plonk basically communicated that smack the carriage return, enter key with gratifying authority to the user who had earned their place in the kill file, not to be heard from again.
The conversation is over, sort of like a block works today.
Edit: See in the definition I linked where plonk is the sound of some poor soul hitting the bottom of a kill file? I think that is debatable, depending on perspective. The peeps who mentored me onto the net at the beginning explained it as that gratifying press of the CR/LF [ENTER] key.
The sentiment is the same though.
---
plonk /excl.,vt./
[Usenet: possibly influenced by British slang `plonk' for cheap booze, or `plonker' for someone behaving stupidly (latter is lit. equivalent to Yiddish `schmuck')] The sound a newbie makes as he falls to the bottom of a kill file. While it originated in the newsgroup talk.bizarre, this term (usually written "plonk") is now (1994) widespread on Usenet as a form of public ridicule.
----
This particular plonk is proper, not just as an insult, which is the general use case, because the person who earned the "plonking" did so in spectacularly stupid fashion, in the opinion of the "plonker."
Total classic!
On some older TTY's, the two asterisks denoted bold text too, here HN uses it for italics.
Plain text would show the asterisks as the linked exchange showed to us.
