Apparently, there are no other reasons that can make a particular website to ban its users other than flagrant flounting of its rules and regulations. This is not only restricted to website alone. It extends down to every other physical community where there are people. If there are no rules and regulations in such community, abuse of certain privileges becomes inevitable.
Same is applicable to websites. Mark you they are also community. And so, if the users decide to deviate from the lay down rules and regulations in using of the site, in order to curb further act of lawlessness by that particular member, and to also serve as a deterrent to other members, the member who breaks the rules and regulations would be banned depending on the gravity of his/her offense. Some sites might decide to revert the ban after sometime while others might not be that generous.