Constant unsolicited phone calls by sales agents

I have two high-value Cell C contracts, both are due for renewal now. I am not interested in renewing because of a very bad experience with Cell C that was not resolved to my satisfaction. (After having had one of the two contracts for about 20 months, they suddenly claimed I had used out of bundle data to the value of R500, streaming a video. I know I did not. I never stream videos on my phones. So I will never sign another contract with Cell C, I do not trust them). Anyway, agents have been calling me almost every day for the past week, every time I tell them I am not interested and that they should stop calling. Today they called me for a fifth time. This is harassment.