Deducting money from my account for no reason

I have cancelled my membership with Bonitas Medical Aid end of Feb 2015. I have received a bill for outstanding payment for treatment received during Jan/Feb 2015. I have paid the said account in the amount of R587-74 during Sept 2015, where after I have forwarded proof of payment to Bonitas Medical Aid. Then I received a statement from them confirming my balance with them is R0.00. Today, FOR NO REASON, the amount of R587-74 was deducted from my account! I don't owe you any money, and nobody gave you permission to deduct money from my account! I also keep receiving sms's from Creditworx, who apparently handles your outstanding payments. They keep sending me sms's, and they keep phoning me. I have forwarded my proof of payment to them also on several occasions, and confirmed telephonically that I have already settled my outstanding account. STILL THEY DON'T GET THE MESSAGE! Please wake up and get your admin in order!!!!! I WANT MY MONEY BACK!!!