Worst service ever from mtn regarding my paid up letter

I had a sim contract with mtn and the contract ended. They kept on debiting my account even though it had ended. I stopped the debit order because I have contacted them several times but no one canceled it . They handed my account saying that I owe them money and that has affected my credit score, I have now paid the money that they say I owe them and I want a paid up letter but they can't assist me with it. I have phoned them but they just transfer me from pillar to post and gave me the email address where I have sent emails and I haven't received a response. I want my paid up letter so that I can clear my name is that too much to ask for? I will never ever be with mtn again never