This should wipe out all cookies from your domain on closing.
window.onbeforeunload = function(){
var cookies = document.cookie.split(";");
for (var i = 0; i < cookies.length; i++) {
var cookie = cookies[i];
var eqPos = cookie.indexOf("=");
var name = eqPos > -1 ? cookie.substr(0, eqPos) : cookie;
document.cookie = name + "=;expires=Thu, 01 Jan 1970 00:00:00 GMT";
}
}
Source:
http://stackoverflow.com/questions/179355/clearing-all-cookies-with-javascript[
^]
EDIT: Just wrap that in a
<script type="text/javascript"></script>
tag anywhere on the page.