MTN is treating me unfairly

I paid early cancellation and outstanding amount from MTN to cancel my contract since I’m out of employment and have no money to pay for contract.MYN had given me final notice and cancellation of subscriber contract.I made payment and sent them proof of payment but they did not cancel my contract.Now they are making me run from pillar to post and they sent me a new invoice to pay. I’m fed up with MTN .