Cancelled service in October 2016, still receiving phones calls about bills

I canceled my service with Greencom in October 2016. Everything seemed fine until February 2017, where they suddenly debited my account for no reason. I had to pay my bank a fee to stop and reverse the debit order. Everything once again seemed to operate fine but then today, 24th August 2017, I received a phone call that the is an outstanding balance for the month of August. Luckily I have the stopped debit-order in place from the last incident otherwise they would have tried to ***** my money again.