The last (and to my knowledge, only) time a filter like this was implemented was in response to the massive Korean advertising spam that happened a while back. This situation was unique in that it both rendered the forums unusable, and was far too much work for the forum/user mods to handle alone. Yes, KYM has a spambot issue, but it has always had this issue. It's most prevalent in the early morning hours, when diligent mods (read: Arcanine and TwilitLord) would sweep every hour or so, but is nearly unnoticeable during daytime.
So yeah, it's a problem, but also incredibly benign and already handled well enough. The larger issue lies in the bots that opt not to post to the forums, but instead create elaborate user profiles. They're easily identified by humans, but can go unnoticed for months if nobody is looking. Like this one I just found to make a point.
It'd be pretty hard to filter all those out, and we can always make the interns trained monkeys moderators handle it. A bit of extra scrolling in the thread index won't hurt you for now.
Obligatory "James pls fix".