[Freddie Bingham]

This will exclude users with userid 1, 5 and 7. Note the portion in bold that you would want to change.

// get top referrer
if ($usereferrer) {
  $ref = $DB_site->query_first("SELECT COUNT(*) AS referrals, user.username, user.userid FROM user AS users
                   LEFT JOIN user ON (users.referrerid = user.userid)
                   WHERE users.referrerid <> 0 AND
                   user.userid NOT IN (1,5,7)
                   GROUP BY users.referrerid
                   ORDER BY referrals DESC
                   LIMIT 1");

  eval("\$topreferrer = \"".gettemplate('topref')."\";");
}

And template:

Top referrer of this forum: <b><a href="member.php?s=$session[sessionhash]&action=getinfo&userid=$ref[userid]">$ref[username]</a></b> with <b>$ref[referrals]</b> referrals.