This WordPress plugin will remove posts from your loop that belong to sub categories.
By default, if you are looking at “Category A” archive, you’ll see all posts from “Category A” as well as all posts from any sub-categories of “Category A”. Once activated, this plugin will only display posts from the current category.
There are currently no options. If you think of any and that could be used by many, I’ll add them to it. For now, just install and activate.