[livibetter]

This is quick solution but not perfect. Find <?php while ( bb_forum() ) : ?> in front-page.php and replace it with

<?php while ( bb_forum() ) : global $forum; if ($forum->forum_parent != 0) continue;?>

It only hides the subforums, so visitors have to click on a top level forum to see if there is any subforum within the top level forum.