Hello,

To exclude a category from the ‘More posts’ section you need to modify the query by which it’s being formed in single.php (the $args array around line 62). Here are the possible category parameters described in the Codex.

So, for instance, to exclude a category with the id of 20 you would do the following:

$args = array(
 'post_type' => 'post',
 'post__not_in' => array(get_the_ID()),
 'posts_per_page' => 2,
 'category__not_in' => array( 20 ),
);

Please mind that it’s best to perform such modifications in a child theme, so that they do not get overwritten during a theme update.

I’m glad it worked out for you.

As for the shortcodes, there are no shortcodes created by this theme. What you saw might have been the [gallery] shortcode (which comes with WordPress) or shortcodes created in your content by a previous theme.