CONTRACT NOT CANCELLED AFTER 3 MONTHS OF TRYING

I took out a contract 2 years ago and never miss a payment. The contract expired and I requested it be cancelled as I no longer require the service. I have sent through my cancellation form and ID documents on numerous occasions over the past 3 months, only to receive an inflated invoice at the end of every month. Their reasoning behind this is that because I am no longer on the contract the account has reverted to a month to month and I now no longer receive a discounted rate. So effectively they are charging me even more for a contract I have been trying to cancel for 3 months? Sounds a lot like THEFT to me... Every time I call the call centre I am told the matter has been escalated, but they can't do anything from their side and that someone would contact me to confirm the account has been cancelled - I am still waiting for that call... and don't even get me started on their retail stores... Probably time to change my service provider