Unauthorized debit orders

I joined this gym last year December 08 at protea glen mall. I paid R100 cash as a joint fee to a guy named Thando and he gave me a receipt. As of February, this company has been deducting extra R50 in my account. I would reverse them and it got to a point that 3 weeks back I received an SMS from them saying my account is overdue and threatening to ruin my name. I called them and was redirected to Bara gym company. They told me that I owe R100 for joining fee hence they've been deductung extra R50 in my account and I told them that I had paid that money and still has receipts. They asked me to bring it to their offices at Bara and I did. A lady with short hair told me that my joining fee was not captured on the system. She made a copy of that receipt and promised me that they will fix it. That was 3 weeks ago. Today they've just took R90 from account again for the joining fee. Am really frustrated by this. They are doing as they please in my account and I even regret joining them. I never had such problems from Virgin Active. Am sure that as soon as I reverse the money a threatening SMS will be sent again because of people who refuse to do their jobs.