Remove Category From Home Page Loop

We recently updated our Pitch Premium theme to Version: 2.3.1. With the previous version of the theme I had created a child theme that helped us filter out a specific category (newsletter archive posts) from the home page loop that displayed ‘Launch News’ posts.

This was achieved by creating a modified loop-home.php within our Pitch child theme which contained the following snippet:

<?php if ( is_home() ) { query_posts($query_string . '&cat=-18'); } ?>

This appears to no longer work with the updated parent Pitch theme. Could you give me any pointers on how to achieve the exclusion of posts from category 18 from the home page loop with the updated Pitch premium theme?