I'm using following code to display only parent category products and remove the child category products displayed in the catalog:

function exclude_product_cat_children($wp_query) {
if ( isset ( $wp_query->query_vars['product_cat'] ) && $wp_query->is_main_query()) {
    $wp_query->set('tax_query', array( 
                                    array (
                                        'taxonomy' => 'product_cat',
                                        'field' => 'slug',
                                        'terms' => $wp_query->query_vars['product_cat'],
                                        'include_children' => false
                                    ) 
                                 )
    );
  }
}  
add_filter('pre_get_posts', 'exclude_product_cat_children');

If you now select the parent category in the catalog, only the products from this category are displayed.

The code works as it should except for one small flaw:
The number of products per catalog page is no longer correct and varies from page to page depending on how many products of the subcategory were hidden.

I could not find any further information about this problem although this code seems to be used more often.