For years a large bank hounded me for a debt that had been resolved years before. They hired a collection agency that scheduled and canceled court hearings repeatedly. I finally hired a lawyer and they quit bothering me. Then, about three months ago, I got a 1099R from them claiming it was a bad debt, which means the IRS will be coming after me for “imputed” income. I called another attorney and he said there’s nothing to keep them from doing this as often as they want.
Find out who signs those letters and have a nice sit down chat with them. You should be able to sue for harassment.
Get a lawyer to go with you directly to the IRS requesting a voluntary audit of the matter...the IRS I think might actually be “helpful” since you are requesting their help. If the debt was settled legally as you say then the IRS can include that in your record. They might even start looking at the company issuing those 1099Rs to see if they are attempting to commit federal fraud! After all, they like taking down the bigger companies who might have to cough up more dough...not penny ante stuff like yours.