Contract expired yet still taking payments

My contract expired with this company on 23 December 2014. I ported to another service provider on the very same day. Autopgae are still deducting money from my bank account via debit order.<br> The Northgate branch continued to send me sms's to upgrade my contract several times since that date despite me responding with \stop"and calling the branch to ask them to stop sending me sms's. I received several calls from them to request an upgrade despite me telling them on many occasions that I no longer require their services. I called them to complain last month and to let them that they should stop deducting money from account. They are still taking money from me. I logged a complaint on their website and no one has bothered to get back to me. <br> The result that I require is for them to STOP taking my money and to refund all the money that they took after the cancellation of my contract."""