DEBIT ORDER CANCELLATION

During December 2014 I purchased an item of furniture from Coricraft Montana and signed up for a 6 months no interest card purchase with RCS. I was obliged to complete a form submitting my banking details. Subsequently I have corresponded with RCS to cancel the debit order and do a monthly EFT payment in order to settle the outstanding amount prior to the 6 month period. My request has been ignored. Every month I do an EFT payment and then an additional amount is deducted from my current account. Each month the amount is different and nobody can tell me why and nobody answers my queries per e-mail as to how I go about cancelling the debit order and my request to stop deducting monies from my banking account. E-mails are sent daily with NO reaction! This month the amount deducted was double to last month and besides this an EFT payment was done well before the required cut off date!