They continue to overcharge me instead of paying back me.

They debited R730 instead of R599 last month as agreed on my contract and instead of reversing my R130.43 now they changing the story from saying am paying for two decoder now they say they must charge me R50 more for pro rata which was reversed to me which is lie, I never received R50 from multichoice. If possible can we cancel the contact Cos instead of enjoying the benefit am struggling financially.