How to NOT display subcategory posts when viewing category

I have a category ‘about us’ which has two subcategories, ‘history’ and ‘staff’.

each of these has a single post in it. When I view ‘history’, or ‘staff’, the correct post is shown, but when I view ‘About Us’, all three posts are shown.

How can I make it so that ‘About Us’ only shows its own posts, and not those of its subcategories?

I’ve got my category_list working correctly now, thanks to http://wordpress.org/support/topic.php?id=22345, and various other posts, but I can’t see the answer to this one.

stef