If the listing is still running just use the option in the drop down box alongside the item on your selling page, or the link on the Site Map page in the middle column.

You should also add the person to your blocked bidder list.

If you only sell within Australia why not set your preferences to block countries to which you dont post?

Log into your My eBay and click/hover on the Account tab at the top, then click on Site Preferences.

Look for Selling Preferences >>> Postage Preferences click Show on the right, then click Edit next to "Exclude postage locations from your listings" and select your blocks.

Tick the box next to "Apply to all current live listings" if you have listings currently running.