Billing non existing contract

A had 36 months contract with C-Track(https://ctrack.com/za/) from 2012 to 2015. From 2015 it was running on month to month. In February 2020 I sold the car. I called them to cancel the contract before my debit order, I couldn't get hold of them. My debit order went through for February. I somehow managed to find their cancellation email address in March. My contract was cancelled in March before my debit order could go through. They went on to debit my account in the name of one month notice period. I reversed the debit order. In April I was charged once again and my account was handed over to their legal department. I have been with them for 8 years without a single claim and a single call from them even when the battery was disconnected from the car. If you are using them, I advise you to just stay away from them.