Mtn doesn't want to cancel my contract.the have put me in errears and I paid the full amount for 36 months

I have took a contract with mtn for 36 months n I have paid them full amount without skiping any months my contract ended December 2023.now I keep get massages from different number saying I must pay mtn ,my account is in arrears,I went to mtn stores da said I'm owing R650 I must pay n call them ,I did dat da cancel it and gave me reference now I still get the massage that I'm owing mtn n is still active mind you I dnt even have da SIM card .the have blocked the SIM card but I must continue paying even if I dnt want their service.the are literally forcing me to take contract.i have called the for the past 2 month no luck.