There are 2 possible solutions:
1. The plugin was suited to an exclusive basis, so there is an option to exclude all other categories, and leave only the one you need. You can do that on the Advanced Options -> Exclude results panel, with the Exclude posts by categories option.
In this case however you will always have to exclude new categories after creation, which might be a bit inconvenient.
2. Another 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.Best,
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