Harresment of Pensioner with a SABC TV Licence account with just interest on interest since 2016

This company is the debt collector for the bankrupt SABC. My mother is a pensioner. Her account is paid up. She however still gets a statement daily for interest on interest on interest on interest since 2016. She gets pestered daily. This nonsense must stop. They must rather get the guys that doesn't pay TV Licences at all!!! If they dont stop they will hear from my lawyer. None of their email addresses word, not even the contact form on the website. They keep on calling and mailing although they know the facts. You can write all the interest back as it will not be paid!!!!!