Debiting my Bank Account after contract terminated

I had a cellphone contract (No. ***4) with Cell C for 6 years which I terminated on 13 March 2015 after giving Cell C one month's written notice (I have now changed to Vodacom). Payments were made monthly by Debit order from my FNB A/C, the last one being for R154.00 on 28 February 2015 and I have asked Cell C in writing and by phone to send me an invoice for the period 1-13 March 2015. They have not done so but instead have claimed R128.93 from my FNB A/C (which cannot be the correct amount for 13 days). I have twice placed a STOP request with my Bank and twice the payment has been reversed but FNB cannot permanently stop the Debit request if Cell C use a different reference number (which they have). I have told Cell C that I will pay the amount that I owe for 1-13 March 2015 in cash when they send me a detailed invoice but they have not done so. I have also told them to discontinue claiming payments from my Bank but they have ignored that request too. Anything that you can do will be much appreciated. Thank you.