OPT OUT ISSUES

I receive post from the Nedbank Personal loans department, which I would like to stop. The letters state that you can phone the call centre to opt-out from receiving the letters and to quote the reference number on the letter, however when I did this the call centre agent wanted my ID number, which is personal information that I do not feel I want to share in order to unsubscribe from unsolicited mail. He could not further my request without me supplying my ID. I find this totally unacceptable.