There’s a setting for this on a per-category basis.
Head over to the category page. For example…
https://meta.discourse.org/c/support/6
then click on the wrench icon here
This takes you to the settings for that specific category. From there, click here.
and scroll down a bit until you see this setting
change that to “no subcategories” like so
And that will give you the desired result for that category - the topic list for that category will only show topics in the parent category and will ignore topics in subcategories.
If you want other category pages to have the same behavior, repeat the process for those categories. Like I mentioned above. This is a per-category setting.