Netstar tracker

I have a problem with Netstar . Can I kindly be assisted with this issue where our 3 years tracker contract with them has expired on the August month of 2019 . They continued debiting my account without calling and doing an upgrade for my tracker. 24 October I called and a cancellation and all went well even got a cancellation letter saying tracker will not be liable for anything after the 24 November . But yesterday they double debited my account,Now I spoke to Stanley Goliath whom I called to enquire about the debit order ,he was rude and even cut off my call and pretended as if he had an interruption because of the questions I asked him. He went on to even threaten me about being blacklisted if I go do a reversal on my bank . Whereas our contract ended on the 30 August. I can't afford to be blacklisted whereas I haven't claimed in the last 3 years and they did not bother to call and tell me I need to an upgrade . I dont understand the debit order that's happening cos the guy I spoke to for cancellation said nothing about the debit order . .