[Erwin]

Assuming you have vB 2.x.x

In newthread.php AND newreply.php (DO THIS IN BOTH FILES),

Find:

PHP Code:

$DB_site->query("UPDATE user SET
          ".iif ($foruminfo[countposts],"posts=posts+1,","")."
          $dotitle"."lastpost='".time()."' WHERE userid='$bbuserinfo[userid]'"); 


Replace with:

PHP Code:

if (!in_array($bbuserinfo['usergroupid'], array(8,10,20))) {
$DB_site->query("UPDATE user SET
          ".iif ($foruminfo[countposts],"posts=posts+1,","")."
          $dotitle"."lastpost='".time()."' WHERE userid='$bbuserinfo[userid]'");
} 


Replace the numbers in brackets with the usergroupids you want to turn post counter off.

Not tested but should work...