Do not list children_cat posts in category view?

I’ve searched a lot, but couldn’t find a solution.
I have a home.php which works with query_posts and all is OK.
I have in the sidebar a category list showing only parent categories even if they have children categories.
Now when I click a category name in the sidebar/menu it will use the index.php to show
a) all the posts from that category
b) all the posts from (all) children categories
What I’d like to achieve: to have listed/shown ONLY the posts from the parent category (that was clicked) and none of the posts from its children categories.
I know I could use a category.php and exclude/include only the posts I want, but it would make my life easier if I can get this for every category.
TIA