Not all robots follow these rules but at least most of the more popular and legit do. You will still most likely get a few bad hits on it, and about the only other thing you can do is add:
if ( !( $logged['admin'] = member_auth( 1, false ) ) )
if ( !( $logged['member'] = member_auth( 0, true ) ) )
if ( !( $logged['aff'] = member_auth( 2, false )) )
$logged['moderator'] = member_auth( 3, false );

Which prevents a nonmember from using the page, but since it already has:
$logged['member'] see more = member_auth(0, false);

You could try:$logged['member'] = member_auth(0, true);

Which will block non member access too.

Something to play with anyway. And your valid members could still abuse it, but it would most likely cut it down considerably.
gameutopia