Incorrect billing and still not resolved

Always had a contact of R359 with mtn and In April this year I upgraded for 369 capped and this time it was minutes note airtime. To my suprice two months down the line I noticed I was debited ridiculous amounts and when I enquired at the branch they said they did cap my account but due to system error it became open line and they promised my account would be adjusted and will get credit but to this day every month mtn is collecting money in m,y acccont and call centre cannot help and they agents at the store keep saying they have sent a query to headoffice to credit me. PLEASE help and is there a way I can cut ties with mtn.Its not my fault that I used their data and airtime cause I was under the impresiiomn that mtn did their job and I am using minutes allocated to me since I was even new to this minutes thing.