Culligan billing for service they do not provide

I used to have water softener exchange service with Culligan. The local dealer decided to stop providing that service as of November 30, 2006. However the Carol Stream, IL, Culligan office continues to bill me for that service. They have not responded to written complaints, one of which I sent certified mail so I have proof of delivery. They have just sent a notice threatening to place my account with a collections agency.