Bleachfag here.
My thought would be if you want to get rid of the worst of those fans, like depicted in your picture OP, then stop the Cartoon Network. That's where the worst of them come from.
Kingdom Hearts? Meh, there's not much to be done about them.
But, honestly, while the little dubfans irritate me, I don't really give a damn what anyone cosplays. In my 10 years of con attendance, I've seen the cosplay focus fluctuate so many times--the major cosplay seen switching from one thing to the next (2003-2004 I saw more Chiis from Chobits than I have seen Narutos combined).
So, if you want to just be lazy about it...wait a few years for the fad to die. Or go to a different con---Nekocon's Bleach photoshoot last year had all of 10 people, to maybe the 12-15 Bleach cosplayers at the entire con.
tl;dr--It's a cosplay fad, and cosplay fads like this have been at con since the beginnings. Sooner or later, the next fad will take hold, and these will end. Wait.
