Constant harrassment from Vodacom

Hardly a week goes by that I don't get a call from Vodacom or someone purporting to be Vodacom offering me upgrades to my contract, changes to the contract and God knows what else. Yesterday, I had had it up to my back teeth. I got a call from a Telkom line with the Durban prefix (031) from someone claiming to be from Vodacom. When I queried why he wasn't calling from a Vodacom number, he claimed that he was in a call centre(?). He was offering some sort of package on one of the numbers that I pay for. I accused him of being a *********** and put the phone done. But I've had enough of these constant calls, so I called Vodacom's Call Centre and asked that they put on their system that I do not get called - I will come to them if I want to change my package or contracts. The lady I spoke to confirmed that she had marked me as "No Marketing Calls" on their system. So that was it, or so I thought! The very next day, I have received another call from Vodacom offering me an upgrade! How is this possible the day after I asked them never to call me. It just ****es you off to be constantly harrassed by a company, that I already pay a fortune to! I'm also meant to be a so-called "Red" client, which I gather offers you extra benefits - well, all I get from them is harrassment. Unfortunately, in South Africa, I think all our mobile operators are the same, so changing service providers is another hassle that I could do without, so I'm stuck with Vodacom, I guess. But if someone can tell me a foolproof way to get them to stop harrassing me, as one of their loyal clients since the mid-1990's, I would be very grateful!