There are many reasons I can think of for banning someone from this site. Inflammatory remarks about race or sexual orientation, "trolls" from rival teams just stirring up trouble, annoying and persistent pictures of felines, posting pictures of obscene or disturbing images of corpses, etc. etc. etc.
I would like to propose another reason. Mental retardation.
Not the type of mental illness which only people in bad taste can make fun of. I mean the type of retardation of individuals that make ridiculous assertions, arguments, and questions that cause those unfortunate enough to read them to have a significant drop in his or her intelligence quotient as a direct result. Sometimes, the stupidity can be so high as to cause a dramatic spike in blood pressure, endangering the lives of some of the readership at risk of heart attack or stroke.
Maybe the Banhammer is too severe. However, some type of warning would be nice before reading a post by a complete moron. The pure lunacy/idiocy of the post requires a response, and oftentimes one would choose not to read a post by a suspected idiot, if only one had noticed the author of the post before reading it.