Actually, it has happened that a moderator has edited, locked, or removed a thread or post. However, it is not something that is taken lightly, and all attempts are made to be reasonable and fair in the exercise of power by moderators here.
Obviously, threads and/or posts dealing with certain issues will always be handled immediately when a moderator becomes aware of the problem. That's where you folks come in -- if you see a post that is clearly against the rules of the forum community here, click the "Alert" link at the bottom of the page. This will send an e-mail notification of the problem as described by you to all forum moderators. One of us will then look at the thread/post and determine whether or not to act on it.
Not all "alerted" post/threads will have action taken, but every "Alert" is looked into...
We do have to remove the occasional post from a troll, or some deliberate commercial spammer, but it usually happens before most members even see it. We watch the forums pretty closely.
The Moderators of this Forum are of the highest caliber. Their answers to questions are always given with a great deal of thought and research. If they don't know the answer to a question they hand it off to a colleague who does. If they make an incorrect answer they are the first to apologize. They give the same consideration to the deletion of a post. We are indeed fortunate to have these dedicated volunteers helping us to solve problems and correct mistakes when we screw up our computers.
Having the greatest bunch of members on the internet doesn't hurt either, IMHO.