Cancelled contract with Netstar

I signed a contract with Netstar a couple of years ago, I think in 2004/05. I continued with them for many years then I noticed that netstar is not delivering,the trekker was not picking or locating my car,I informed then,they did nothing. Later I cancelled the deal with them and later I sold the car. They responded saying that they received my cancellation letter and confirmed that the contract is cancelled. Now I keep receiving calls almost everyday from them,they demand payment from me. I have been telling the agents that the contract was cancelled long time ago but they don't stop. Another agent requested the letter I received from Netstar, I asked her to give me an email address but she gave an incorrect email address. Every time I request the email address they keep telling me that they cannot email it to me.I just want to know why should I pay for a service that is long cancelled.