over charged

I had been getting a very bad service from mondo i took a phone contract for R285 and wifi for R259 but to my surprise month end they deducted R900 and the following Monday they demanded R7000 i called they to lodge a complain more than 10 times but still got no help as we speak the balance has escalated to R15000 within a space of 5 months i have been battling with this company to resolve this matter for so long with no positive feedback i just want to pay off the amount we have agreed upon i don't know what to do anymore