MTN won’t stop taking money from my account for a laptop that never worked!!!

In January this year I received a call a lady offering me a laptop deal from MTN. Laptop was delivered beginning of February and was not working I went to mtn in Sandton City to get this sorted but they connected me with a company called platinum warranty which would collect the laptop from me because the store wouldn’t take it for the reason that it was an online purchase. Platinum warranty collected the laptop and after a few days gave me a credit note number to give to mtn online I did this but mtn online gave me a run around I asked them to cancel the contract they don’t want to do that I’ve been paying since February for a laptop that I don’t have I call mtn almost every day but I never get assisted I just want mtn to stop taking my money! I don’t know what to do anymore