Under 47 USC §230(c), the website is immune from suit for defamation, unless you can prove that the website actively conspired with the poster to produce the defamatory statements. This means that your sole recourse is against the poster.
A defamation action can potentially be brought in the court where the plaintiff is located, if the defendant can be shown to actively market his or her services in the plaintiff's jurisdiction. So, you could sue locally, and the defendant may have to defend in your court, which would give you huge leverage to obtain a settlement.
Otherwise, you will have to sue where the defendant is located, and that will be expensive.
You could sue in small claims for the maximum $7,500, which would save you the cost of hiring a lawyer, and maybe that would provide an incentive for the defendant to remove the posts. Generally, before suing, you would send the defendant a demand for retraction, as a means of demonstrating that you attempted to mitigate your damages.
Hope this helps.
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