Exclude sub sub categories from displaying in the archive page

Question

I have the following category structure:

Category x >> Category x Sub >> Category x Sub Sub 

In my index.php file, I have created links to category x and so on.

So when the user clicks on the link in the main page, they go this page: wordpress/category/categoryx/

Only posts published under category x sub should be displayed here, the problem is that in this page, posts published under category x sub sub are also displayed.

In my archive.php I used :

  <?php while ( have_posts() ) : the_post(); if (in_category($cat)) { ?>
  ....
  <?php } endwhile; ?>

this correctly hides the categoy x sub sub posts in the main category but it also hides them from their own sub category.

So in this page: wordpress/category/categoryx/category-x-sub/

the posts published under category x sub sub are hidden too.

Answer 1

/* exclude sons */
$_categories = get_categories(array( 'child_of' => 16));
$exclude = '';
foreach($_categories as $category){
    $_exclude .= '-'.$category->term_id.',';
}
$_exclude = substr($_exclude,0,-1);
$_cats = $cat.','.$_exclude;
query_posts(array( 'cat' => $_cats ));
while ( have_posts() ) : the_post(); 
    ......
endwhile;