Terminix paid bill but turned over to collection agency , Internet

Terminix billed us repeatedly for a service for which we already paid, and then turned us over to a collection agency.  

We telephoned Terminix after each billing to explain that the bill was paid and they agreed our balance was $0.00.  Yet Terminix continued to send bills.  Today we received notice from a collection agency that our account was turned over to collections and that we have 30 days to pay.  

Here is the situation:  In our 2012 annual house inspection Terminix claimed there was a fungus underneath our house. We could not find the fungus.  We hired another inspector and he could not find the fungus.  Our house was under contract to be sold and this false report of fungus scared the buyer.  In order not to lose the sale of the house we agreed to have the house treated for the non-existant fungus, and to put in a new vapor barrier that Terminix claimed we needed even though we already had a vapor-barrier in place.

We paid Terminix in full the $800.00 on August 27, 2012 for the fungus treatment and vapor barrier.   Terminix required this full payment before they would spray the house.  They sprayed the house on August 28, 2012.

Terminix continued to send us bills for the Vapor barrier and the Fungus Treatment.  We called the company each time we got a bill and explained that the bill was paid.  They agreed that our balance was zero and claimed they would not bill us again.

We were extremely upset today to receive notice from Nationwide Credit,Inc that our Terminix account was turned over to this collection agency.  We called Terminix and a representative named Darlene said we would have to call the collection agency and straighten this out ourselves.  We then called the Terminix corporate headquarters and have not received a reply. 

Out credit rating is being ruined by Terminix over a bill we paid before they even rendered service.  Why are we suffering because of Terminix's mistake?