Ridicilous attempts to prevent me from cancelling

Terrible customer service unless you're giving them more money. I have been trying to reverse an upgrade since the day I agreed to it (Monday 26 October, no device included). Since then I have spoken to at least five different people, all of whom have either made promises they have failed to keep or given me excuse after excuse as to why they can't process my request. These excuses included: ‘I don't have access to the old system so I can't see your account’ (How can you have agents who are literally incapable of customer service?), ‘the cancellations department only handles cancellations at the end of contracts’ (What about cancellations at any other time, where does the customer go then?), and ‘only the person who processed the upgrade can reverse it’ (this honestly makes it sound like some kind of magic spell rather than a simple change of data in a system). Somehow one of these people, who apparently couldn't access my account, migrated me from a Smart S package to a Smart Top Up 2GB package after I stated that I wanted to reverse the upgrade, cancel the account and go prepaid (as in buy airtime and data as needed. I don't know how they got that confused). At least two people have told me they would escalate the issue. Only one out of the five people have actually bothered to call me back as they all promised to do. Nobody can tell me if my request to reverse the upgrade has been lodged because they 'can't access my account'. I finally actually got a query opened and received a reference number on the 4th of November, but 8 days later my account is still listed under the Smart Top Up 2GB plan, and I was debited for this account on the 10th. I am getting very frustrated. Every agent I've spoken to either brushes me off with false promises or says they can't help me. What am I supposed to do?