Vodacom Call Centre harassment

I am sick and tired of getting calls from Vodacom call Centre every single day about migration on my account. It is harassment! I keep on saying I am not interested but the calls keep on coming every day. The call Centre staff are not thinking about the customer but their own **** pockets, it's all about commission that they can earn and not saving you money. I want this harassment to stop. I am not interested it tying myself to an extended contract for another 36 months as I am a pensioner and cannot afford it. What must I do to stop these calls on a daily basis???????????