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How do you handle troublemakers on a forum?

Sometimes hurt people start hurting everyone else.

That perspective is a good place to start.  If you start with believing that the person causing the trouble is probably not doing so out of pure maliciousness, then you will treat them more appropriately. 

With that foundation, you can do a few things....

First, contact that person. By phone if possible, email if you need to. Your tone with them needs to be person to person, not admin with a stick. Very often, just this personal interaction makes them snap into realization that your community is not an impersonal machine. You ask them personally to stop posting those things as a personal favor. Maybe invite them to express their concerns over email in private for a while. They may perceive that as even better than posting to the public. They have the attention of someone inside and are getting heard.

Second, you can very tactfully post after them. Something like "Bob, we have discussed this issue in private and I asked you not to post here. I am taking your concern seriously, but this isn't the place." What this does is illustrate to the rest of the community that you are being gracious and he is being disrespectful.

Then... if it continues... you may have to ban him. Maybe even ban him and post a low key apology for banning him in the community.

If you do have to ban him, the community will think "what a great guy that admin is" and the troublemaker is  discredited, having shown everyone his rudeness.  If the troublemaker listens (and about 90–95% do), then you have shown another human being respect, and kept him in your community.