I've posted a demo site, so that it is easier to follow along and better understand my question. Please take a look here:
http://www.slingerlandweb.com/modalbox

I am using the Scriptaculus script called ModalBox. I'm using an onLoad event to load an external page on all three pages, so no matter what page you come into the site on you will see the pop-up. Here is the part that I'm having trouble with. I only want the pop-up to display once throughout the whole site. After the first time it pops up any time you change pages the pop-up should not display again.

Is there anyway to set cookies to check if the pop-up script has already been run, and if so do not run it again? What would be the best way to do this? If anyone has any ideas I would greatly appreciate it. Thanks.