Plonk is a Usenet slang term for adding a particular person to one's kill file. It can be used as a verb to describe the action, or appended to the end of one's Usenet post to publicly demonstrate that you have added a person to your kill file.

Example usage:

> Anonymous User wrote:
>
>   Your ideas are foolish and poorly conceived. 

That does it!

*plonk*
In this case the respondent has added Anonymous User to their killfile and has decided to emphasise the point by adding "plonk" to the bottom of his post.

The word is supposed to denote the sound of the user hitting the killfile (like hitting the bottom of a barrel). It is commonly used as a verb: "I plonked that idiot".

It can also mean Put Lamer on Killfile.

Plonk is also British slang for cheap wine.