Hi!
On the back-end you can configure the plugin to exclude certain categories, but it’s not the most convenient solution, because if you add new categories, you will always have to exclude those as well. (Advanced Options -> Exclude results panel) For older plugin versions (pre 4.9.3) this is the only option.
Plugin version 4.9.3 and further:
A better solution is to put the following code to the functions.php file in your current theme directory:
All you need to do is change the $category_id variable to the category ID you need.