Cell C spamming me with phone calls for unwanted upgrade

I keep getting calls from Cell C's "upgrade department" on a DAILY basis. I have a simple small data bundle for a tablet, and I DON'T WANT TO UPGRADE. Some consultants told me they'd put a note on my account saying I'm not interested in an upgrade. Other consultants told me there's no facility to create such a note. I have told them MANY times I don't want to upgrade.

Must I cancel the contract to be able to not upgrade? I'm perfectly willing to go that route and get my 5th Vodacom contract. I've only had issues with Cell C from day 1 (7 years or so ago when I took out the contract I had to pay for some premium SMS junk that I was supposedly subscribed to with that number. My wife uses the SIM on an iPad. It doesn't not have SMS capability to explain HOW that happened please).

What do I do so that Cell C stops phoning me EVERY DAY?