Beware of unfair business practices Mtn

I had a Data contract with Mtn and the contract came to an end on the 11th of May and Mtn kept on billing me. I called their useless call centre to advise that the contract is expired and they should stop billing me as I am no longer using their data because the contract has expired. They advised that I should have called and cancelled and upon asking them to put me through the contract division, I spoke to a gentlemen and asked him that since the contract came to an end, I no longer wish to continue. He said I must pay the money for the service which thdy billed , after the contract expired and I said its fine, I will pay in instalments until its paid up but they must cancel the contract and stop loading data on the sim which the guy say its fine ( mind you I was paying 269 pm and after the contract ended, they charged 900 pm) They continued charging me 900 until today.