Send a certified letter with receipts. If you have an attorney relative or friend who will give you legal office letterhead with a firm’s name on it, that’ll help.

Demonstrate that the bill is an error and you will refer further attempts to defraud you to your attorney and pursue damages to the fullest extent possible due to their continued attempts to extort you. Citing some laws and federal oversight mentioned in this thread wouldn’t hurt.

I had Jordan Tax Services send me thousands in bills for taxes I’d already paid and they didn’t cease until I wrote them to basically say, “Here is proof you are wrong. Harass me with fraudulent bills at your financial peril.”