Impossible Cell C Direct Marketing Opt-out

I keep on getting spammed with sales and marketing SMSes and calls from Cell C even though I have tried unsubscribing several times. The SMSes don't even allow you to opt-out of marketing, which I can't imagine is legal.

I have also asked probably more than ten times during marketing calls from Cell C for the agents to opt me out for their marketing since I basically feel harassed by all the phone calls and SMSes at this stage, just for each agent to tell me thy will opt me out and for the marketing calls to just continue.

I asked the last agent whom I spoke to two weeks ago to escalate the matter to her team leader and opt me out, but after that I received SPAM calls from them every second day as well as SMSes.

I also tried the opt-out service on their website, which was followed by a call from an agent telling me that they're following up regarding the query I submitted on the website showing interest in a contract.

It really shouldn't be legal for it to be impossible to opt out of this harassment! I am beyond fed up and wanting to change my number and never deal with Cell C again just to stop getting all the annoying and unsolicited SMSes and calls!