Well, it seems given the way the mod works, that changing the code of the base forum will change the code of all the forums created from it, so I'm trying to change the code of the base forum so that all the other forums will automatically disallow a guest from reading the forums. I did this successfully for turning off new registrations, but it was a lot more straightforward.
I think all I need to do is edit a number somewhere in this code, but I don't even have an idea of what; short of trial and error.
<?php
$result = $db->query('SELECT g.g_id, g.g_title, g.g_read_board, g.g_post_replies, g.g_post_topics, fp.read_forum, fp.post_replies, fp.post_topics FROM '.$db->prefix.'groups AS g LEFT JOIN '.$db->prefix.'forum_perms AS fp ON (g.g_id=fp.group_id AND fp.forum_id='.$forum_id.') WHERE g.g_id!='.PUN_ADMIN.' ORDER BY g.g_id') or error('Unable to fetch group forum permission list', __FILE__, __LINE__, $db->error());
while ($cur_perm = $db->fetch_assoc($result))
{
$read_forum = ($cur_perm['read_forum'] != '0') ? true : false;
$post_replies = (($cur_perm['g_post_replies'] == '0' && $cur_perm['post_replies'] == '1') || ($cur_perm['g_post_replies'] == '1' && $cur_perm['post_replies'] != '0')) ? true : false;
$post_topics = (($cur_perm['g_post_topics'] == '0' && $cur_perm['post_topics'] == '1') || ($cur_perm['g_post_topics'] == '1' && $cur_perm['post_topics'] != '0')) ? true : false;
// Determine if the current sittings differ from the default or not
$read_forum_def = ($cur_perm['read_forum'] == '0') ? false : true;
$post_replies_def = (($post_replies && $cur_perm['g_post_replies'] == '0') || (!$post_replies && ($cur_perm['g_post_replies'] == '' || $cur_perm['g_post_replies'] == '1'))) ? false : true;
$post_topics_def = (($post_topics && $cur_perm['g_post_topics'] == '0') || (!$post_topics && ($cur_perm['g_post_topics'] == '' || $cur_perm['g_post_topics'] == '1'))) ? false : true;
?>
<tr>
<th class="atcl"><?php echo pun_htmlspecialchars($cur_perm['g_title']) ?></th>
<td<?php if (!$read_forum_def) echo ' class="nodefault"'; ?>>
<input type="hidden" name="read_forum_old[<?php echo $cur_perm['g_id'] ?>]" value="<?php echo ($read_forum) ? '1' : '0'; ?>" />
<input type="checkbox" name="read_forum_new[<?php echo $cur_perm['g_id'] ?>]" value="0"<?php echo ($read_forum) ? ' checked="checked"' : ''; ?><?php echo ($cur_perm['g_read_board'] == '0') ? ' disabled="disabled"' : ''; ?> />
</td>