lwm debt recovery

they keep contacting me looking for someone i dont know i phoned them now telling them that i dont know the person in question. their responce was we will keep contacting you until we get our money. first they were looking for a white male and this morning they are looking for a black female. both times its regarding fnb homeloans. they even gave me the peoples accountnumbers at fnb after i told them they have the wrong number. is this even legal