Cancelled my contract but you still billing

I had a 24 contract with Mtn, the contract ended April 2017. I contact Mtn as they kept on sending sms advising my to cancel or they'll keep on deducting, I contacted them to terminate the contract and the lady said they'll be a last deduction end of may, after last deduction end of may I contacted Mtn to request a settlement letter and the agent said it's already cancelled she can see on the system but she can send the letter. End of June, Mtn deducted as usual that when I stop the debit order. Last Friday 09 Feb 2018 I found that Mtn handed my account to their "legal department ", I tried calling Call Centre 3 times and the agents said they can't help as the account is already at legal department . They gave an email ********** I sent a mail but no respond.

What must I do NOW. I need this to be sorted very Urgent.