MTN Disaapointing data contract service

Hi i had Data contract with mtn via call centre and it had reach it's ending date. When it came to month to month, i requested several time to be change to prepaid. However MTN call centre keep on making a fool out of me stating that they will change it but i always get debited for something i really dont need. Im so disappointed at MTN for binding me on a contract that has been exhausted. Kindly assist for i am really not happy with MTN poor service. Thank you