Is this just spam emails or an individual bothering you? Either way your options are limited on Optus email. I would recommend switching your email to a non RSP dependent one (you lose your email if you ever leave Optus). SOmething like GMail is free and its pretty easy these days to port emails across over the course of a few months. You get better filtering and security and control over blacklists etc.
The added advantage is that your unwanted emailer won't know your new address.