Set cookie to disable popups

Thanks for the Boxzilla plugin, which I’m using to display a mailchimp subscription form (via the MC4WP plugin). I have another way for users to subscribe to my newsletter, i.e. not via the popup. How can I programatically (JavaScript) prevent the popup from ever showing again, once that happens? Obviously I need to set a cookie value; I know how to do that. But what cookie, with what value? Many thanks.