I am harassed by Hybrid Collections due to Netstar's negligence and carelessness

I have had my Netstar account since April 2011 and I was quite happy until June this year. I sold my car end May and wrote to @Netstar that I am no longer needing the tracker because the owner had his own tracking company. I received an acknowledgement email with reference number: CAS-***-W6R4. Of course I knew I would have to pay end June because any and every company probably needs a full month for such cancellations. Middle of July (in the middle of celebrating my birthday) I get a call from Hybrid Collections to notify me that my account with Netstar has been handed over to them. Which account because I cancelled? Only to find out last week, after over 10 calls from Hybrid, that my account is still active at Neststar. How when I no longer drive that car for over 5 months now!!! I am now told I owe Hybrid Collections money and that my name has been sent to Credit Bureau. How is this possible when I have cancelled? I screamed to at least 5 consultants from Hybrid and told them to cancel immediately. One said the cancellation was through, then immediately after the phonecall I received an SMS that I had agreed to an arrangement to pay x amount by end of the month. This is pure harassment and it is really getting me frustrated! I plain refuse to pay for a service I no longer make use of, especially when I did cancel!!! Please do something asap.