Payday-Loan-Yes Nightmare

I have been trying to dig myself out of the payday loan trap for the past couple of months. I have closed my bank account and sent letters to all PDLs revoking ACH authorization, C&D letters, etc. I have overpaid on most of them, including Payday-Loan-Yes. My loan was for $350, and I have paid a total of $680. I received a call at my work the other day (even though the letters I've sent told them not to contact me at work.) I got her name and called her back after work. The lady says I owe over $400. I told her I had already overpaid them, and they should be issuing me a refund! I don't think they're legal in Ohio, but even if I give them the $15/$100 allowed in Ohio, I've still overpaid. She just wanted to argue with me. She then kept asking me if I was refusing to pay. I told her I wasn't refusing to pay, but that I had already OVERPAID them. She then told me that she was going to mark my account as a refusal to pay and that it would be sent to a collection agency. She then abruptly hung up on me. Needless to say, this really annoyed me, so I proceeded to call her back about 20 times and left her messages each time on her voice mail. She never picked up, but I hope she had a good time checking her voice mails from me demanding a refund for my overpayment. Of course I've not heard back from her since.

But if it does get sent to a collection agency, how do I deal with the collection agency since they don't know that I've overpaid. They just see the balance that the PDL says I owe.