Difficulty cancelling contract that is coming to an end.

I have a data contract for the past two years and have since installed an alternative internet solution at home. The data contract is only 24 months and I requested that it be cancelled when the term ends, so far Vodacom has refused to cancel the contract when I called in and instead wants me to downgrade it. I have gone in store to a consultant who confirmed that it can indeed be cancelled and that there should be no reason why they refuse to cancel it over the phone. I fail to understand how this can be so difficult to cancel a contract that is coming to an end.