Charged for ended contract.

I had a contract with Vodacom which was suppose to end in May 2024. Before the end of the month they call me for an upgrade and they gave me a new device .Seeing that I will not afford , I returned it sealed never opened and requested that they cancel the contract they never cancelled my contract, yet they took their device. up until today they are deducting the money this is the fourth month paying for sweet nothing. Each time I call they tell me it will be sorted in 7 days but still do nothing. I am getting depressed I feel like they taking advantage of me and they are robbing me. I need help. I have called many times it seems as if they don't want to help me. and through this I may never wish anyone to take their services if they will treat you like this.