MTN unauthorised subscription from account.

Every month I automatically get notification about been charged between 2 and 10 rand a day, for MTN daily deals… I have not subscribed to this, but the onus is then on me to ensure I cancel this subscription. Surely this is *******, and is **********? How to I prevent MTN from continuing this, as phone calls do not help, nor visiting the branches, it’s been going on for a year now.