Mtn doesn't want to cancel my contract

I took a pocket router contract with mtn last year November. Bearly had the router for a few days and it was giving me problems. I then called their call centre to cancel the contract and they said I should return the router to any mtn store. When I took it to the store they didn't want to take it saying I should take it to a different store. Which I did and they still didn't want to take it.

This is like two weeks of having this router, I decided to stop using it. First debit was in December upon the agreed amount and date but guess what, they debited more than the agreed amount which I reversed hoping that they will call me to fix that but nothing.

Waited for their call in January but nothing. February I was called saying I owe over R1300 which I asked them how because if it was the data that I used for two weeks cannot accumulate that much. Asked them to explain how did the money accumulate, they failed to answer. And all of this has affected my ITC record.

Early July I settled the account and I have been calling and writing to them to cancel my contract with no help. It is now ruining my job opportunities. I just want them to cancel the contract because it's settled.