Write them a letter explaining that you're paid off and enclose copies of your title (which should reflect no lienholders), the receipt reflecting the payoff, and the copy of the check for overpayment. Keep a copy for your records and send it certified mail RRR. This should get them to back off. If it doesn't, you may need to consult my firm or another consumer protection lawyer. You can find one here: www.naca.net. Good luck.
The above is for informational purposes only and does not establish an attorney-client relationship.