******* - Adding Subscription on my account

From January I have been charged for some Airtime subscription which I never asked for nor knew anything about. I wasn't even receiving statements, but when eventually got them to send me one I noticed this *******.<br> I called the call centre and after being given another number to call I was told someone would call me back.<br> This never happened so I went into the store to query the matter. They called the same call centre and gave me a reference number. A month later the charge was still there, so I went back into the store to query. Once again they called in and I was given another reference number. In JULY (six months later) they stopped the charge and reversed one months charge. What about the other 5 months ?<br> Who gives them the authority to add these charges on your account.