Will not remove our number from data base

For some reason Nedbank has our land line listed on their system as belonging to a gentleman we have never heard of. Despite REPEATED requests, they have not removed our number from their system and they continue to call several times a day (today they have called four - 4 - times! So far...). During each call we clearly tell them that Mr X is unknown to us. During each call we are told that our details will be removed, only to be called again and again and again. Our requests to speak to a call centre manager have been denied - not always politely. This is clear harassment on the part of Nedbank and I would like someone in a position of authority to arrange for the incessant calls to stop.