MTN sponging on me

On 3rd of January 2021 I upgraded with MTN for R299 contract and amazingly so I'm now getting charged more than R1000 every month from what they say is "Self Service " which I haven't use before and apparently when I investigated further I came to realize that it has been happening since 2020 but it started with small amounts. I reported the matter early this to MTN and asking them to cancel this Self service thing because I had to pay R1400 on that particular number and now it has resurfaced again. I just want out of MTN