Actually, there is a setting for this, although it appears to have been lost over time from non-use. You can put it back in by editing the adm-options.php script and adding the code in bold to lines 239:
Code:
Content visible to verified customers only.
I'll add this back into the codebase; so if you don't want to make the file edit, download the current build and upload the updated adm-options.php file.