Does anyone have a followup letter for validation?

I have sent a certified letter to NCO Financial detailing to them that they need to validate the account which is for medical services. The total amount of the bill is $50, however, I only took one injection and paid the physician office. We went back and forth, I called the billing office, they advised me 2 years ago that they would take the information out of their system showing I did not owe the payment. I continued to get billing notices from Caremark and I called them several times telling them I am not going to pay because first, I paid the physicians office and I did not take the 2nd injection which was billed at $25. In the interim, I had the surgery I needed so the 2nd injection was not necessary. NCO mailed back a copy of the last billing notice from Caremark dated last November 2009 and a letter telling me that this was the information that I had requested which it is not. They never answered my question of whether they were licensed in my state to collect, proof that I owe this money, the agreement I established with the company showing my signature, etc. Now I need a followup letter and I am not exactly sure how to word this to get these people off my back.

Any suggestions would be greatly appreciated.