MTN is robbing me

I was offered a contract by MTN for a laptop which I thought was a great deal. But when I received the laptop it was super slow. I then returned the laptop and cancelled the contract within 7days. Well it’s been 3 months later and they are still debiting my account with R4000 for a contract that I don’t even have!! This is *****. Please don’t accept any offers from MTN. It seems they take your money and delay cancelling and this leaves you with no money and a bad credit record!