Mtn is a ****

I have been trying to cancel my data contract with mtn because of poor internet connection for Months last year. When I finally got them on the phone they told me I have to pay a cancellation fee . I continued paying until my contract ended in Jan but they are still charging me for data that I haven’t used from last year and u can’t cancel from the Branch!!!! U have to call but they blocked my SIM card so how am I supposed to cancel my contract . They keep on deducting money from my account so I stopped it and now my credit score is affected