How to Protect Registration Page

ok i did it / opened register.php and wrote the following
session_start();

if(!@$_SESSION["UserID"])

{

$_SESSION["MyURL"]=$_SERVER["SCRIPT_NAME"]."?".$_SERVER["QUERY_STRING"];

header("Location: login.php?message=expired");

return;
}

Hi ,
i need to protect the Registration Page , so only already registered users can have access to create a new account user.
so no unathorised users have access to registration page
pls help