Harassed by Cartrack tracking company

I am being harassed by cartrack. Last month February I sold my car and as a sult I did not need thier service anymore for tracker. When I was cancelling my contract they told me about cancellation fee of R2 650.00 that I need to pay. They also gave me the option of paying this amount over 4 months (arrangement). Ever since that day I have been getting calls almost every day from Cartrack call centre telling me that I am owing and I keep telling them that I am not owing ,I have never missed not even one payment. The payment arrangement was arranged to start at the end of this month (March), last month they debited my account with my premium amount even though I had cancelled my contract. Today I received a letter of demand as if I am refusing to pay. Please guys help me, I do not have peace anymore. These things will affect my credit score.

Please help me. I am being harassed by cartrack, no matter how much I explained my situation and asked them to fix thier system and remove me from thier call list, they dont want to do that. I do not have peace.