Unless there comments are libellous or offensive, I don't see whatright a moderator has to ban anyone. The quality of the posting should not be an issue.
A moderator can ban who they like, as can the sites administrators. Why wouldn't they be able to, if they feel it is necessary? Is there some legal precedent you can point towards which would indicate that they don't have the 'right' to do so?
As for 'quality of content', if a member's only contribution to a forum is to post drivel which doesn't contribute to debate and discussion in any positive way whatsoever, then I'm guessing a moderator is quite within their 'rights' to impose a ban. I think the offence is called 'trolling".
Edited by TheTerminator, 13 August 2014 - 03:28 PM.