What can we do if our former employer/company is harassing us for resigning because of their illegal activities?

My former employer, A**E Inc, has recently started losing contracts for engaging in dishonest business practices by newly appointed management. Once my colleagues and I discovered information about dishonest business practices, **we resigned from the company**. Now, A**E Inc, is harassing me (and my colleagues who resigned) by send me **multiple cease and desist letters** through a contracted attorney. None of the allegations are true and hence they (will) have no proof for their allegations. I got in touch with attorneys and they recommended I don't answer these myself but hire them to start/stop the allegations by sending a response that basically says "please show proof and my client has not and will not engage in any activity that will make you concerned" Since there are multiple letters, attorneys are quoting me from $5k - $7k for their services. None of the allegations are true and hence my former employer has no proof for their allegations. They are trying to intimidate me and cause me financial harm (indirectly) by sending these cease and desist letters Should I go ahead and take a personal loan of $5k - $7k to pay the attorneys to send a response **or** should I draft a 4 line email/letter as response to each such mail that says: "Receipt of your cease and desist letter on such and such day is acknowledged. The allegations made are concerning and evidence of the same is requested to resolve the matter. I advise you to assure your client, just as I assured A**E Inc management on the day of my resignation, that I will not engage in any way that is against the employee contract and T&C I signed when I joined A**E Inc. I am an honest professional and I take great pains to move myself away from places of business that are run in a dishonest manner. Please respond within 14 business days with supporting evidence of the allegations made, if any.