MTN Contract Cancellation

I actually need some help or advise please. I have been trying for weeks to cancel my MTN contract but to no avail. Only feedback you get when going to a branch is that you need to call customer service to cancel. When calling you go through all the steps but once you get to the prompt of cancelling, the call is either ended or the line is disrupted, etc. I am at that point where I just want to block their debit order - maybe they will get the picture then. Their product offerings are just bad - I currently have 8G data that I cannot use because 4G are for YouTube and 4G for music which I will never use - I never signed up for these buckets of data and would like to cancel both my contracts asap - any advice welcome!