Why banishment?

Since I have not yet had to banish anyone from participation here, I have been thinking about why so many blogs and forum heads insist on banishing their adversaries. Pharyngula, Uncommon Descent, Panda’s Thumb and many other blogs and forums ban comments from adversaries and often erect barriers making it very difficult to register. P.Z. Myers of Pharyngula periodically complains about how his “security has been penetrated,” sounding more and more like Captain Queeg of the Caine Mutiny. He has another revealing tactic. He will allow comments to be sent from anyone but only he decides which will ever appear. None of mine have. I had to resort to subterfuge to comment on his blog only to see my comments disappear within minutes.

DaveScot/David Springer of Uncommon Descent lets dissenters speak only then typically to dismiss their author with “___ is no longer with us” or “you’re outta here Homo” or some other equally charming farewell. Then if things get dull he may let them back in again so he can enjoy expelling them for a second and in my case a third time. My first banishment from Uncommon Descent consisted of being unable to reregister. I  have no idea who was responsible for that one. Most probably it was Dembski. Of course banishment remains the responsibility of the blog owner or owners.

ARN and RichardDawkins.net have not only banished me but denied me viewing their proceedings from this computer.

Why do so many blogs and forums  find it necessary to engage in such tactics? That is the subject of this thread. What does this mean? What does it accomplish?  How can one properly respond to a critic if he cannot continue the dialogue to some sort of conclusion? Doesn’t it just promote resentment? Thankfully I have not yet had to delete a single comment or ban any user from this blog. Of course that may change. I look forward to the views of others on what I regard, with anonymity, as a very important issue. In the meantime –

“Will you walk into my parlour?” said the spider to the fly;
T’is the prettiest little parlour that ever you did spy.”

Mary Howitt