to clear a cookie, you use the set cookie behavior and set the expiration date to a date in the past.
this code is an example of how to clear the password cookie if the remember me button is unchecked and the login form submited:
<?php
if ((isset($_POST["imageField_x"])&&(!isset($_POST["Rememberme"])) && $_POST["imageField_x"] != "")) {
setcookie("password", "", time()+(60*60*24*30), "/", "", 0);
}
?>