Paying for a cancelled service????

What a struggle to get the contract cancelled. They require a calendar months notice, so I gave notice on the 2nd of March (the first business day of the month), this equals a calendar month, but not according to them. At the end of March I received an invoice for April, after going back and forth for about 3 days, as they did not see the whole of March as a calendar month, they finally agreed to cancel my contract. So they cancelled the contract on the first of April, but they never cancelled the debit order from their side, I have been asking for a refund since cancellation but all my emails are being ignored. How is it possible to pay for a service which I cancelled, I seriously don't understand how they expect me to pay for April if they are no longer the service providers.