BAD RAIN SERVICE

My father who is a pensioner signed up for Rain fiber , their internet connection was bad and when he would log a complain they told him they would fix it , for months and months he kept complaining ,he then requested for them to cancel his subscription cause he was not able to use their internet they refused yet kept debiting for a service he was not getting , he then changed his banking details now they are harassing him for a service they have not been providing him with

please help