Directory / plugin
Just One Category
v1.1 · by Michael Adams (mdawaffe)
On a category's archive page, displays only the posts directly in that category, not in any sub-categories.
About this plugin
Just One Category will cause WordPress to display on a category archive page
only those posts which are direct members of that category, i.e. this plugin
will exclude from that category page all posts which belong to any of that
category’s subcategories but do not belong to the category itself.