What would you like to know about this please?
Ok. So what is your question about this?
Did you say anything about a refund and telling everybody how bad the food was?
It is always very difficult to prove what was said.
There is no such thing as verbal blackmail. A blackmail is a blackmail whether reduced to writing or not. It is possible to argue in academic law that a threat to tell everybody something that would be damaging to a person in exchange for financial gain is a blackmail. It is not one the police would be interested in but it is blackmail in academic law.
To take action against him you would have to do so in defamation. That is quite hard to do in this instance essentially because you are anonymous on these review sites and so it cannot be damaging to your reputation. If it is not damaging then there is no defamation. Action ends.
If you can get over that hurdle then you would have to show on the balance of probabilities that you didnt say what he says. That would probably be easier to do that you think as he won't have made notes of the conversation so it might well be possible to shake him during the course of cross examination
But the anonymity is a problem.
You could always get a solicitor to write to him warning him of the risk of a defamation action if he does not remove it or edit it. That would cost about £100 plus VAT and may work.
Can I clarify anything for you?
Usually defamation is not worth the cost.
A letter before action can be though