Yes, this can be done, but it requires changes to some different files including a core VirtueMart class. You'll have to change the function "get_availability_data" in the class "ps_product" which is in the file "ps_product.php" to ignore if it's a parent or not. Then you'll have to change "availability.tpl.php" to determine if it is a parent, and then to not show the quantity in stock (since the quantity for the parent is meaningless).