Unfair blacklisting by Discovery health

I used to work for a bank and was on the Bankmed medical aid, which is being administered by Discovery Health. I left the bank in January 2017, which the meant I cancelled my Bankmed medical aid. When I left bankmed I owed then R736.62, which I paid in April last year. But lo-and-behold, I've just discovered that Discovery Health blacklisted me for this money - as I'm talking now I have a default in my name from Discovery. I've tried calling their call center since last week when I discovered this, but to date there is no resolution in sight.