The miniportal actually changes your main.tpl file... which means you cannot do what you require...
I had a similar situation with a previous site I done... Here is a work around...
Comments if this is good or bad from the PunBB professionals would be appreciated.
header.php
replace...
else
$tpl_main = file_get_contents(PUN_ROOT.'include/template/main.tpl');
with...
else if ($style=="forum")
$tpl_main = file_get_contents(PUN_ROOT.'include/template/main2.tpl');
else
$tpl_main = file_get_contents(PUN_ROOT.'include/template/main.tpl');
forum.php, viewtopic.php, viewforum.php
find...
require PUN_ROOT.'include/common.php';
after add...
Now you need to get an unaltered main.tpl then rename it to main2.tpl and upload it to /include/template...
Hey presto.... forum will now will be without menu.
Side Note: Any other pages you require without the left side bar just put the
as done in forum.php, viewtopic.php and viewforum.php.....
Dont think I have missed anything...