DEBIT IN PLACE

How can I stop being harrased by these people. Firstly I gave them instructions in NOVEMBER 2011 to deduct an amount from my account......in about April/May 2012 payments stopped so I assumed by c/card was paid......then in Dec last year I get a phone call from them saying I still owe on the c/card and that I put a stop on the payment.....I have asked them for proof of this stop payment and to date no body has responded to my request...any help with this please