How do you delete all the cookies for the current domain using JavaScript?
As far as I know there's no way to do a blanket delete of any cookie set on the domain. You can clear a cookie if you know the name and if the script is on the same domain as the cookie.
You can set the value to empty and the expiration date to somewhere in the past:
var mydate = new Date();
mydate.setTime(mydate.getTime() - 1);
document.cookie = "username=; expires=" + mydate.toGMTString();
There's an excellent article here on manipulating cookies using javascript.