According to Section 230 of the Communications Decency Act, the website itself is not liable for third party content. So someone in your situation has to go after him if the website refuses to delete his post, or, is non-responsive. What someone in your situation can do is threaten to sue him for defamation, which is defined as 1) a false statement purporting to be fact; 2) publication or communication of that statement to a third person; 3) fault amounting to at least negligence; and 4) damages, or some harm caused to the person or entity who is the subject of the statement.
What you can do is send a takedown demand letter using an attorney to this individual, via certified mail. This normally resolves the issue quickly.
However, if he already took down the post, then there is nothing to demand. You can either actually still SUE him for the damage done under defamation, however.
But you cannot really control other people's speech, unless it is directly defaming you. You can try to send like letters to individuals commenting if they are libeling you and you know their real name, but that can have a Streisand Effect, so I would be careful.
So you can get him to take off the posit - and it seems he did. You can sue him for defamation if you'd like. But you cannot hold the website accountable, and it would be a very problematic effort to control other people's reactions to the (now deleted) post.
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