Plunk
Ever get really annoyed at Arcanist Braedin? Well, at least just want him to shut up? Then "/plunk arcanist braedin"! Yay! No more spam.
Should work on just about any NPC or player, though it was really intended for the former. Main feature is that it took me less time to write than my fruitless search for these specific capabilities in an up-to-date addon.
Other features include: silencing achievement spam, foreign "language" spam, and whatever else I find convenient.
Besides "/plunk" and "/unplunk" there is a simple Interface Options configuration panel.
I know, but I needed something to test the general idea with and that seemed convenient.
It's seriously not really extra overhead and very little extra code. Please don't lose sleep over it.
I'm curious why you chose to add extra code and incur additional overhead to suppress achievement messages and guild achievement messages when these can be turned off more effectively in the default UI by right-clicking on the chat tab, selecting Settings, and unchecking the checkboxes next to these message types?