put this code at line 1:
<?
if(isset($_POST['UserEmail']) && $_POST['UserEmail'] != "") {
$emailArray = explode("@", $_POST['UserEmail']);
if($emailArray[1] == "xyz.com"){
header("location: ".$_SERVER['PHP_SELF']."?dom=no");
}
}
?>
this will not allow the domain xyz.com to register.