Cancelled contract not honoured

Years ago I took out a contract with Autopage, a decision I regret till this day. I silently rejoiced when they folded last year because of the ineptitude of their staff across all levels up to management. Then my contract was taken over by Glocell. My nightmare began. They no longer sent itemised billing and when they called me earlier this year to upgrade my contract I declined and told them that I want to cancel my contract and was put through to the relevant department. My contract with Autopage had been from month to month, because it had long expired. My contract with Glocell was cancelled in April. But from May they have been debiting my account. Worst of all they can't even present any evidence that I have used their service after April. They are now sending me threatening letters that they will list my name on the credit bureaux. I don't know how they have the temerity to tell me that they will suspend my services , a whole five months after I cancelled the contract. Why is it okay for companies to continue to conduct their business in this ********** manner? Which part of cancelling the contract, do people that work at Glocell not understand? Why are they pretending like they are not aware that my number has NEVER been in service for the last five months? Now I am going to be blacklisted just because I cancelled a contract that should have expired long ago, but had always been propped up by the manipulative month to month arrangement?