1. I might suggest that if your customers are having difficulty navigating your category structure that you come up with another way to display your categories so your customers can find them more intuitively. ie: if the current menu system isn't suitable for your customers, use another one that is.

2. To force the categories_ul_generator to process only 1 level instead of 6, change the $max_level from 6 to 1 in categories_ul_generator.php:
Code:
class zen_categories_ul_generator {
  var $root_category_id = 0,
      $max_level = 6,