Overcharged on my bill for past 7 months

I have never come across a more useless company than Telkom!!! Firstly, I app**** for a 4MB uncapped internet line in July last year that was advertised at R599pm. I signed up and only after receiving my first invoice in September did I learn that an agent accidentally put me on a 10MB line instead of the initial 4MB that I signed up for. I queried the costs and was downgraded to the 4MB line but am still currently being charged as if i am on the 10MB line. i asked that they debit on the last day of every month for a charge of R600pm from my bank account to cover the charge of R599pm. i am actually paying them R1 more each month. Since September, each month, i have had to contact them and query the high costs that incur on my account EACH MONTH. I have countless reference numbers and SMS's saying that a dispute has been made on my behalf. But each month the costs just increase and i see NO improvement. Yesterday i received my invoice for January month and am now R3459.31 in arrears to be exact. My internet connection has been suspended twice, once last year and once in January this year. I had to phone again to get my internet line back on and query the overcharge. I have never missed a payment Since July of 2017 but no matter how much times i pay and query my account, the old amounts do not disappear from my invoices. Also, when i received my invoice in September last year , i was being billed for 3 months instead of the 1 month of August. I'm afraid that these costs are giving me a bad credit rating and could affect any future purchases, such as a house or car. I HAVE HAD ENOUGH!! PLEASE HELP