MTN REFUSAL TO CLOSE EXPIRED CONTRACT THAT IS PAID IN FULL , ADDITIONAL R400 CHARGE PAID UP/

MTN and it's Retention Dep refusal to close a Expired Contract even after Numerous phone calls asking them to Close Contract and furnish me with Paid up letter, They have ADDED R400 charge on a closed Contract and they Refusing to Reverse it . It's a NIGHTMARE to CLOSE A PAID CONTRACT as they using ******* TACTICS TO RETAIN CUSTOMERS