joe1972

I helped myself by introducing the children=0parameter to wp-includes/categeroy-template.php file inside the function function wp_list_categories($args = ''). I placed the following code snippet pretty at the top of the function (after the line $r = wp_parse_args( $args, $defaults );):

if ( isset($r['children']) ) {
		if($r['children'] == 0) {
			$query = "SELECT w.term_id AS id FROM wp_terms w, wp_term_taxonomy t
						WHERE t.term_id = w.term_id AND
						t.taxonomy = 'category' AND
						t.parent = 0 ORDER BY w.name";
			$tlc_ids = $wpdb->get_results($query);
			foreach($tlc_ids as $c) {
				$tlcs[] = get_category($c->id);
			} /* now we have all top level category objects. */

			/* Which is the active category? */
			if($_SERVER['REQUEST_METHOD'] == 'GET') {
				if(isset($_GET[cat])) {
					$current_cat_id = $_GET[cat];
				}
				else {
					$current_cat_id = NULL;
				}

				/* Let's mark the active top level category. */
				foreach($tlcs as $cat) {
					if("{$cat->term_id}" == $current_cat_id) {
						$cat->active = true;
					}
					else {
						$cat->active=false;
					}
					$res[]=$cat;
				}
				$tlcs = $res;
			}
			return $tlcs;
		}
	}

It may not be the best solution but it does the job for me: Just gives me all top level categories only!