Boy, I really need to save a copy of this explanation one of these days, I'm getting tired of re-writing it:
Warrior Cats as a fandom was banned because of the conduct of the fandom. Several fans of the books came to the site en masse and began writing several stories per day which were all below general site standards; such as extremely poor quality, glitching / inability to complete the game, dead ends, missing links, repetition, cliched dialogue, exc. Some stories also contained content that the CYS community found objectionable, such as the main cat character being bound and raped, so on.
A story below site standards has to be removed or unpublished, regardless of the topic or author.
As a result of the flood of stories, CYS became one of the top results for WC on Google, and became even more overrun. Meanwhile, the WC fans were demanding special treatment because they felt that anyone criticizing their stories or unpublishing them were 'haters' who just didn't 'get' the books or were trying to stifle their creativity, apparently, even though we were judging them the same as everyone else.
Barring a few trolls, senior members did try to help the WC fans improve their quality of writing in order to meet site standards and stay published. I was one of them. For my efforts, I was cussed at, cursed, insulted, dismissed as yet another hater, or summarily ignored.
That kind of treatment and willful refusal to follow the same standards as we all follow eventually bred contempt, which led to forum wars and general unrest on the site. Peace was attempted on several occasions and other solutions to fix the problem were tested, but failed. One of the site admins eventually settled on a temporary blanket ban which, considering it's the only thing that even remotely helped, is probably never going to be lifted.