Harassment form NUDebt for a MTN account that was resolved long ago

Nu Debt Management has been harassing me for more than a year now. I told them from the beginning that I took a dispute against MTN to the Ombudsman. The Ombudsman settled the matter and I received a apologies and a paid up letter form MTN. I send this information several times to the Nu Debt people on the email addresses they gave me. But they are still harassing me threatening me with Credit Bureau listings and so forth. ***. Can you please stop harassing me now?