Currently there isn't a way to only show it if the user is not logged in. However, you can do a bit of php work and make the contents of the box different for those that are logged in compared to those that aren't logged in. You'd use this for the contents:
$body = <<<EOF
<?php
if (\$user['USER_DISPLAY_NAME']) {
echo <<<STUFF
This will display for someone who is logged in.
STUFF;
} else {
echo <<<STUFF
This will display for someone who isn't logged in.
STUFF;
}
?>
EOF;