Discovery Credit Card won't let me close my account

I have been trying to pay up and close my credit card with Discovery since April 2020. I have emailed them multiple times but never receive a response. When calling, I get re-directed multiple times before I eventually get cut off. I finally got hold of someone who sounded vaguely sensible, who assured me I would receive a response in writing within 48 hours... It's been a week since. I am now getting sms's saying that I have an outstanding bill with them which is even more infuriating!

I used to have a discovery credit card through FNB, but since they have gone out on their own, trying to get in contact with anyone is impossible.

What do I have to do to close an account with these people?