When is Matrix going to stop taking my money...

A company I would never want to have any dealings with again. I have been arguing with for with 3 years now. I cancelled my contract a few years ago. But I had to pay the contract out. Now it's paid. Because last year May they told me in an Email I owed R 1800, and they told me I owed an extra month, witch I checked and I have paid. I get my new invoice and they tell me I owe R350, how much longer are they going to ***** from me. When is it going to stop