MTN CHARGING ME *****ULENTLY

MTN is charging me *****ulently. I have completed my contract term & asked that the terminate it. They don't want to retentions department says I owe R600 which I know nothing about. MTN is stalling closing the account because the next month is coming and they want to charge me again. They are now charging me R300 which I didn't agree to forcing a contract down my throat.