Debt collector harrassing for a bill that was already paid a year ago.

I was a very good client of Bonitas for years in my personal capacity. When I changed jobs, my new company made me change to their medial aid and I had to cancel with Bonitas. Up till then Bonitas was great. Then after I moved, Bonitas sent a final invoice to my old email address that I was not aware of. No call. No SMS. No actual bill. Nothing. I found the invoice 3 months later by coincidence, which I then paid. I thought all was good and well. Now almost a year later Bonitas debt collectors are making my life a nightmare. Harassing me multiple times a day, every day. They won't accept that the bill was paid almost a year ago, Asking for thousands of Rands. And Bonitas just says sorry, nothing they can do. That is ridiculous. And now what? Get harassed until I get sued or pay again?