15
Do like this:
xoops_header();
$badusers = array(12,34,102,45);
if ($xoopsUser) {
if (in_array($xoopsUser->getVar('uid'), $badusers) && $to_userid==ofendeduserID) redirect_header('index.php', 3, _PM_MESSAGEPOSTED);
$myts =& MyTextSanitizer::getInstance();
replace 12,34,102,45... for the bad users IDs.