Bad customer service by Vodacom

I have a Wifi contract with Vodacom which I took in December 2016 which provides me with 20gigs per month. In December 2016 Vodacom invoiced me for an amount close to R6000 and when I enquired; I was told I had exceeded my limit. When I demanded a breakdown of how I accummulated such a huge amount; I recieved an email the following day advising that " as a valued client; my account has been credited and most that amount was written off" My monthly subscription is R300 but Vodacom invoices me for R420. Everytime I question that I am given a different story. I was advised by Vodacom to cancel the debit order and pay my normal subscription whilest my matter is bieng investigated. Vodacom suspended my account in December 2017 due to outstanding amounts yet I was paying my subscription. I made several calls today on an attempt to resolve this matter until their cancellation department advised me to contact their legal section so that I can get the outstanding amount in order fo me the cancell the contract. Legal gave me an amount of R875.38 as an outstanding amount. I transfered that amount so that I can cut my losses and continue with a life without Vodacom. I have just received a call from Legal telling me that since I am cancelling the contract prior to its expiery; I have to pay the full contract amont together with pernalties. I dispute that because I believe Vodacom has forced me to cancel this contract due to their managemant of it. Can somebody please help me out of this situation