Thank you very much for clarifying. First of all, I am sorry to hear about the issues you have experienced in your situation. Whilst reviews by customers are legal and possible, even if they do not place you in a good light, they must be truthful and factually correct. As long as someone is telling the truth, they can say whatever they want, even if it is a damning assessment of your services or conduct.
So the key here is to identify what the obviously incorrect statements are and challenge these. I would not concentrate on any trivial statements which do not have that much of an impact and go after the really serious ones, such as alleged blackmail.
Under law, the review could amount to corporate defamation or malicious falsehood. These basically occur when a false statement has been made about you or your company and it has damaged your reputation, or could do so.
See here for malicious falsehood:
The Defamation Act 2013 is relevant to your request and you can mention that too.
Hopefully, I have answered your query in a way that is simple and easy to understand. If anything remains unclear, I will be more than happy to clarify it for you. In the meantime, thank you once again for using our services.