Poor service and lack of interest

I am appalled at Vodacoms lack of service, I feel they cheat customers from what they due. I had contracts for year with them and my last contract they decided to take back the airtime I paid for, surely there is a law around it... If I buy prepaid airtime do they take it back no, so why should they take back airtime on contracts I rightfully paid for. I called the called center and the rude agent said that's how it works?? If u purchase goods at a retailer and its not used the retailer don't come collect what's not used... Highly annoyed