Unauthorised debit order

I cancelled my debit order many months back by email and by calling the call centre, I find they still carry on debiting me. I eventually have to put a stop on the debit order after phoning them many times to stop debiting. It's been a couple months but now I find they're debiting me again, they don't respond to emails and they don't seem to keep to what they promise to do when you phone them. I would never recommend this company to anyone wanting a data contract, their service is the worst I have seen.