[Logician]

In new thread.php, find:

PHP Code:

$permissions=getpermissions($forumid);
if (!$permissions[canview] or !$permissions[canpostnew]) {
  show_nopermission();
} 


Before that add:

PHP Code:

if ($bbuserinfo['usergroupid']==X AND $bbuserinfo['posts']>=Y)
{
show_nopermission();exit;
} 


(Replace X with usergroup of these users and Y with the max post count allowed)

If you dont want them to reply existing threads too, add the same code to newreply.php before:

PHP Code:

updateuserforum($threadinfo['forumid']); 


Not tested, but it should work..

Logician