osmir

I got my solution of the topic.
Main idea is to filter out posts placed in subcategories. So we can use negative values in cat variable of the query_posts().

My steps:

1. Determinig list of subcategories for current category and merging them together

   $subcategories = get_categories(array('type' => 'post', 'child_of' => $_GET['cat']));
   
   foreach ($subcategories as $i => $value) {
       $excluded_cats .= ",-".$value->cat_ID;
   }

   You’ve got something like ,-10,-5,-3 Note leading comma.

2. Generating cat variable

   if (!empty($excluded_cats)) {
      $MainLoopArgs = array('cat' => $_GET['cat'] . $excluded_cats);
   }
   else {
      $MainLoopArgs = array('cat' => $_GET['cat']);
   }

   I include current category and exclude subcategories in one cat. Result is something like 4,-10,-5,-3.

3. Passing $MainLoopArgs into query_posts()
   query_posts( $MainLoopArgs );

Sorry, maybe my example not so graceful, but it works! 🙂

P.S. Yes, you have to make (m)any validation checks of variables for realy robust app.