Horrible service provider

I have been using MTN since 2008 but they don't treat me like one of their customers. I have 3 contracts with MTN and one ended the 13th of June in January they called me for an upgrade I told them no I don't want to upgrade infact I want them to cancel the contract and they agree to cancel the contract, today I received an invoice with the same installment and Im very confused cause they agreed to cancel my contract..I called them just now and the lady by the name of Lerato gave me an attitude that I was suppose to cancel the contract ...what kind of services is this? now I have to pay them while my contract ended the 13th of June what exactly am I paying for? Useless service provider