Cell C upgrade department harassment

I am being harassed by the upgrade department of Cell C. They phone me up to FIVE TIMES per day. I have had more than FIFTY calls from them in the past couple of months. I blocked the number on my cellphone and then they started to phone me on my landline. I have been into the Rosebank branch to ask for help but that has not stopped them. I do not want an upgrade, I am very happy with the month-to-month contract that I currently have and do not wish to change it. All I want is for them to stop harassing me. Please. My cellphone number is ***.