mtn had been has been debiting extra money

A package of R229 was sold to me, I added insurance for R69 total amount that we agreed on was 300 per month,but ever since MTN is debiting R450 every month on my account for the last 7 months. I went to the store early February to correct the matter, till today MTN can not refund and debit me correctly as promised. Im always getting stores that the store is waiting for feedback, for how long do I have to wait??? I regret buying a phon4 with MTN the service is very poor and slow as if they are doing clients a favour.. I want to cancel this contract, if it takes 3months for such a big service provider to give feedback on a simple query like this. The I seriously give up, ill rather pot to a different network with my contract.