Ok, here is how to exclude. You will have to put the board id's in an array twice.
Find:
// TPortal recent topics block
function TPortal_recentbox()
{
global $scripturl, $context, $settings, $txt, $modSettings, $forum_version;
// is it a number?
if(!is_numeric($context['TPortal']['recentboxnum']))
$context['TPortal']['recentboxnum']='10';
// leave out the recycle board, if any
if(isset($modSettings['recycle_board']))
$bb = $modSettings['recycle_board'];
else
$bb = 0;
$include_boards = null;
$what = ssi_recentTopics($num_recent = $context['TPortal']['recentboxnum'] , $exclude_boards = array($bb), $include_boards, $output_method = 'array');
and replace with
// TPortal recent topics block
function TPortal_recentbox()
{
global $scripturl, $context, $settings, $txt, $modSettings, $forum_version;
// is it a number?
if(!is_numeric($context['TPortal']['recentboxnum']))
$context['TPortal']['recentboxnum']='10';
// leave out the recycle board, if any, to exclude certain boards put board id's in $bb array below
if(isset($modSettings['recycle_board']))
$bb = array($modSettings['recycle_board'],26,70,19);
else
$bb = array(26,70,19);
// to include only certain boards put board id's in array below
$include_boards = array();
$what = ssi_recentTopics($num_recent = $context['TPortal']['recentboxnum'] , $exclude_boards = $bb, $include_boards, $output_method = 'array');
You need to replace the string 26,70,19 in the code with the list of board ID's you wish to exclude. You need to do that in TWO places...