I had to do two things before I shut out overseas buyer completely.

First I blocked all overseas countries in postage settings, then I removed overseas post options from all my listings.  Maybe the second procedure would be sufficient by itself, but if you want to be sure, this worked for me.

This should take you to 'Manage Posting Settings'

     https://www.ebay.com.au/ship/prf

Near the bottom of the page is the option  'Exclude Postage Locations'   Click 'Edit'.

On the page that comes up, just  tick every box under 'International'

After doing that, I still had overseas buyers, and had to remove overseas post options from individual items. Fortunately you can do this in bulk (I can't remember if I could do this before I had a store, but I'll assume so.)

From your  'All Selling' page, tick the box at the top of your listings that selects all items, then click the 'edit' button.

On the subsequent page click 'Edit Fields', and select  'International Postage'.

Choose 'No International Postage'

After you've clicked 'submit',  it will remove whatever overseas postage specifications you have from any items.