Virgin Active Billing and Debt Collector

I had opened up a Virgin Active account about a year and a half ago, when I first opened it up in Edenvale, I went with a sale consultant from the Virgin Active Branch to the bank where we set up a debt order to come out of my account every month, there after signing the contract, with the contract it included a month of gyming free, I fortunately went to gym for the month, there after I acknowledged that the money did not come off my bank account in which I approached the woman at reception to ask about this, she indicated to me that this is a mistake and its being handled, in which I accepted and moved along, after this incident I didn't attend the gym because no money had come out of my account. after this I had moved away from Edenvale to Pretoria. 6-8 months later I was called by MBD telling me my account is in arrears of about R1 400.00. What do I do I want them off my back and to stop harassing me due to something that was not my fault.