[Logician]

Without hack:

You can disable access of the forum to some usergroups. Their search will automatically exclude this forum then.

With hack:

Edit search.php, find:

PHP Code:

if ($action=="getdaily") {
  // get allowable forums:
  $forumsql=getallforumsql();

    if (isset($forumid)) {
        $forums=$DB_site->query("SELECT forumid FROM forum WHERE INSTR(CONCAT(',',parentlist,','),',".addslashes($forumid).",')>0");
        $forumsql.=" AND forumid IN (0";
        while ($forum=$DB_site->fetch_array($forums)) {
            $forumsql.=",$forum[forumid]";
        }
        $forumsql.=") ";
    } 


Replace line

PHP Code:

$forumsql.=",$forum[forumid]"; 


As:

PHP Code:

if ($forum[forumid]!=X) {$forumsql.=",$forum[forumid]";} 


Replace X with the forumid that will be excluded.

Not tested but should work..
Logician