You can't. There is a Block Sender option I believe but its unclear exactly what Optus does when you push that button. If the issue is spam then even though the email address might look the same, it usually isn't behind the scenes anyway.
FWIW I would highly recommend using a third party email account. GMail would be my suggestion. Much better security (Two Level Authentication) better recovery options, spam prevention and an ongoing development team on the product. Plus you don't get tied to one RSP and have to lose your email address anytime you want to leave. Its a lot easier to migrate across while you are still with Optus than after you decide to leave. Anyway just a suggestion.