How about something like this on your functions.php:
add_action('wp_head', 'no_robots_on_uncategorized_posts');
function no_robots_on_uncategorized_posts() {
if(in_category('uncategorized')) {
wp_no_robots();
}
}
This will output the following line of code on the header of your ‘uncategorized’ posts:
<meta name="robots" content="noindex,nofollow" />
What this means is that even though search engines will see the page, they will be told to ignore it and don’t show it on their results.