Bad service from MTN Retention

I paid my 24 month contract in full before the contract expired and was assured the account was settled completely. Yet I’m constantly being charged an extra amount of R696 which is not outstanding or owing. Despite reaching out repeatedly to MTN retentions all I get is a standard response that I’m not on contract and the account is in order. So why am I still being charged this amount? Plus R100 bank charges for the unpaid debit order! Very frustrating experience which taught me a valuable lesson, don’t sign up with a contract at MTN you can’t seem to get out of it! I lost my job and still dutifully contacted them, paid the quote to cancel the contract but struggling to get any help with crediting this wrong amount which keeps going off!