Still, please explain how admins make money from extra traffic? New participants have nothing to do with the forum finances. It's been publicly said that money is earnt from mainly advertising, not new registrants.
SEO wise these bunch of "Shitposters" are giving you enough traffic to let the forum rank-up on google, yahoo, bing, Alexa or any search engine available right now on the web. That means more and more advertiser will flock to this forum to get their product advertised to reach more potential customer because of the FACT the forum is ranked at the topmost of all other competitors.
(Although I don't totally agree to let loose all the shitposters)
Vod is right "only tag accounts as shitposters if it is obvious to even the brain dead fool"