Legally publication of true facts is protected. You refer to your contract, which no one on Avvo has seen. The precise details of what you intend to do and how could change this analysis.
Be aware that the mentally unbalanced normally move on, and by starting a "war" you are likely to prolong this situation. This type of behavior is going to be an epidemic in the 21st century. We have one such person (at least) here on Avvo.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
Mr. Doland is right. A lawyer would have to actually see the post to get a true picture of the case and whether it had merit.
He's also right that having any kind of contact, interaction, or legal argument with a mentally unstable person could lead to more issues for you. I don't have a sense of the grand scheme of the situation from your post. You'd have to hire a lawyer for that although it's certainly a factor to consider. You don't want to be the focal point of the ratings of a lunatic ( I'm not saying this person is a lunatic although I'd certainly be mindful of that possibility).
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If you want the situation to escalate one sure way to do it is what you suggest. It is far better to encourage positive reviews and let customers or clients know, on your website, how seriously you take customer service. Give guarantees and follow through.
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It seems to me that using "creep" and "freak" are very helpful to you in that the reader will generally see the review for what it is.
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