Blacklisted for a contract I have settled in full.

I took a contract with Mondo early this year. Months later i decided to cancel it due to bad service that i was getting. I called them and informed them that i want to cancel and was told of the cancelling fee which i paid in full in May. I sent proof of payment to them but now i am listed by MTN for not paying.

How can they blacklist me for something i already settled and sent proof of payment?

I want my name cleared on ITC as soon as possible.