Account suspended unfairly due to MTN's error

I am outraged by the level of service I have been receiving from MTN. I upgraded my contract last year in August; I opted for a 'My MTN Choice 50' plan for the duration of my post graduate studies. When processing the upgrade at MTN's Queenswood branch, the manager explained and assured me that I'd be charged a fixed amount every month (R80) according to the plan chosen. I also requested a limit be imposed on the charges in order to avoid any additional costs. Since then, 7 months ago, MTN have been overcharging every single month for a promo service fee despite me having kept my previous phone. It has been the most frustrating experience dealing with MTN contacting me and threatening to suspend my account on a monthly basis since last year August! I have revisited the store on countless times (a store whose service ended up in my boyfriend cancelling his MTN contract) and have phoned the 808 support center ad nauseam to no avail. This is ridiculous and completely unacceptable. I chose to temporarily lower my spending, and yet MTN still bill more than what was promised, every single month. I have received countless promises yet nothing has been done to correct this or refund me.