The state of the law in your jurisdiction and competent legal counsel is of utmost importance in formulating a specific response, you never know what kinds of weird lobbied-for state statutes or local ordinances have been bought by creditors' lobbies via graft, it's a sticky legal area with lots of stink of bad law and graft in it. So the following is just a broad, general discussion of one possible response you may consider -after- obtaining jurisdictional knowledge and speaking to local counsel.
Generally, consider sending them a denial letter stating that you deny the debt, did not agree to it in any way, orally or in writing, at any time. Further, you believe that their attempts to assess and collect it are not only generally unlawful but intentionally fraudulent, arise and amount to violations of both state and federal statutes and cases governing debts, contract formation and enforceability, fraud, and other well-settled areas of common and statutory law.
Instruct them clearly to cease all further collection attempts, statements or communications of any nature with you immediately, that any negative reporting to credit bureaus will trigger immediate legal action in the form of a complaint filed in local magistrate/small claims court against them by you, and that the damages claimed in such a suit will far exceed $40. Further, consider stating that any future contacts or attempted contacts addressed to you from their practice will result in bills to their practice from you at the rate of $250 an hour for reading and responding to such attempted contacts. Should be no more than a one page letter. Once more, know the local law before proceeding with any type of response or plan.
Would not post anything negative on review sites, as defamation actions for such are getting more and more common. Generally, it's a good idea not to mention specific people, places, things, businesses in any sort of internet posts going forward unless you can document whatever you are claiming and are prepared to do so.
Good luck.
