unfairly billing my account.

on the 24/11/2015 in Protea Glen mall, me and my sister were approached by a representative Muzi Ngubane from Gym company and asked for our details and we told him that we were not joining the gym and he advised us to fill in our details and will keep them until we're ready to come and join them. On the 01/02/2016 I was debited an amount of R765 by the Gym company I went to my bank to enquire about the debits and was advised that Gym company had debited my account for both me and my sister without my knowledge. When I confronted him in the Mall he told me that we said we want to join the gym and I was the 1st person to complain about joining the gym under false pretence. I am a student and Muzi never asked me for my student number or proof of registration, we didn't discuss the payment date as I'm not working and was never told that my sister will be billed under me. I went to Gym offices at Bara on the 03/03/2016 and was asked to leave my contact details for his Manager to call me back even today I still haven't received his call. I also received an sms today from them that I should pay my account urgently in order to maintain a positive credit record. Please kindly assist me.