Upgrade rejected, refuse to cancel contact on request, but billing after contract expired

I have been a good customer of Cell C for over 4 years, my account is always up to date even in excess as I over pay my account, but Cell C just decided they are done with me and refuse to upgrade my contract. I have been calling for months to do the upgrade, they keep telling me they will get back to me. Since they won't upgrade the contract, can you just cancel it then, they are trying to sell me the minutes and data, not they are sending me the bill for the contract they refuse to cancel before it expired.