Billed for things i didn't buy month after month

i took out a contract for a internet bundle for R139 per month. they said it won't go over the GB limit but did for months. R1600 + and R500 + the first two months. when I went back there I was told I should have asked for a top up regardless that the kid who sold me the contract told me it won't go over and does not work there any more. they told me I can change it free of charge to top up after six months. I went back more than six months later (april 2015) they changed it and said it will be active on top up from June. since june I've been billed R400 - R600+ per month. last month I went back there to complain and a call was logged for a refund. everyday I called the call has not been opened from head office side. Yesterday R422+ has been taken from my account again. Please help!! I've been paying more on this contract that it should've cost me in two years.