(In light of the banning of Robert, I thought I would dust off, and update, this post from 2012.)
TAC is a group blog and each contributor normally makes moderation and banning decisions in regard to their own threads. When the blog started nine years ago I was initially somewhat hesitant to use either moderation or banning but that has changed over the years. Here is some explanation of my current policies regarding both.
Moderation is automatically applied to anyone who has never left a comment at the blog. This of course is a mechanism to prevent drive by trolls from launching a pure insult comment. After the initial comment is approved the commenter is taken off moderation and may post freely.
What gets someone placed on moderation?
Direct insults aimed at another commenter. I normally allow some lee-way if a commenter is directing an insult at me, at least if it is witty, although my patience tends to be limited. A repeat offense may result in a ban.
Outrageous comments. (Yes, T.Shaw I am looking at you!) These include threatening to shoot anyone, a comment filled with vulgarities, etc. I used to have T.Shaw on permanent moderation in this category, although I suspect he rather likes it as it enhances his bad boy of the blog image. Happy to oblige T.Shaw. (I have taken T.Shaw on and off moderation over the years. I think it has become a game with us!)
Anti-Catholic bigotry. This blog was not set up to give anti-Catholics a forum.
Riding a hobby-horse too frequently. Some commenters will insist upon bringing every discussion around to their hobby-horse issue. Do that too frequently and moderation awaits.
Wall of Text. No, you may not engage in text spam that bears little relationship to the subject of a post.
Getting on Don’s nerves. I blog for fun, and commenters who take the fun away will find themselves in moderation.
This is not a comprehensive list, but the above are the major categories.
Banning from the site occurs in the following situations.
Drive by trolls. Normally you do not get to see the comments and I simply ban the authors as a matter of course.
Anti-Catholic bigots.  Banning is for those who either do not take the hint from a stay in moderation, or who make a comment so vilely anti-Catholic that it is a waste of time keeping them around.
Anti-Semites and Anti-Blacks. I do not wish to keep them from their Klan rally.
Disturbed individuals. If a comment indicates to my untrained eye that someone is mentally disturbed they will be banned, mostly since taking verbal potshots at a deranged individual is not sporting.
Conspiracy mongers. If you are certain that the Illuminati, the Tri-Lateralists, the Cattle Mutilators or (insert name of group) are behind the scenes pulling the strings, we will not keep you from sharing your insight on other sites.
Being a persistent pest. Longtime readers may recall the late Catholic Anarchist who was banned after a year’s attempt at turning every thread into a fight between him and all and sundry. That got old fast and it was a violation of the first commandment of blogging: Thou Shalt Not Bore!
Attack trolls. If your goal in life is to start constant fights on the internet, this is not cyber fight club.
F-bombs. Constant use of profanity is boring and will not be tolerated.
Violating the rules against acting crazy on the internet. Go here to this fine post by Paul Zummo to read the rules.
Not a comprehensive list of the factors I take into consideration when banning someone, but most of them. I hope that no one I ban takes it personally or to heart. Banning from a blog to my mind is equivalent to being gummed by an elderly toothless poodle who is attempting to tell you that your presence is no longer desired on her turf. It doesn’t really hurt, but it is time to move on!
Banning and moderation help me prune the comboxes to make them more entertaining to our readers. To me, the comboxes are just as important as my posts, and I pay close attention to the comments as a result.
We have a great stable of regular commenters at TAC. I usually find your comments insightful, frequently witty and sometimes challenging. You have helped make this blog the success it is, and I thank you from the bottom of my cold lawyer’s heart!