[TWTCommish]

Sure. Right BEFORE this...

  if ($bbuserinfo[usergroupid] == 10 && $action == 'delete' && $shoutid) {
    $DB_site->query("DELETE FROM shoutbox WHERE shoutid = $shoutid");
    header("Location: shoutbox.php");
  }

...add this...

  if ($bbuserinfo[userid] == X || $bbuserinfo[userid] == Y) {
    header("Location: index.php");
  }

Replace X and Y with the userids you want to block, and those users will just be redirected back to the board's index when they try to load the Shoutbox.