Unfair payment demands for product i did not ask for.

Took out contract with Vodacom for 2 gigs data @ +-R200p/m on tablet ********** , which commenced 20/12/2014 and expired 08/06/2016. I wrongfully stopped payment as i was moving from Free State to KZN. I started receiving payment demands and called Vodacom to terminate contract, settle account etc. I spoke to Pricilla from Vodacom, cancelled the contract and made arrangements to off the debt. I was given reference no. I ********** 7 and paid off the account in 5 months. I emailed proof of payment and confirmed that the contract was cancelled and all paid up. Now i am receiving payment demands again, Vodacom is now charging me for a month to month contract. How can this be right when i settled my account to cancel the contract? I did not agree to or sign permission for a month to month. I find this very unfair and being a pensioner, +-R500.00 is alot to pay out.