Use this extension to disable/enable cookies on the current site.
It's a simple switch: click to disable, click to enable.
It works on "per host" basis and will disable cookies for one domain only (the site opened in the currently selected tab).
When you disable cookies, it will delete cookies set by this site and will reload the current page. (Auto-reload can be disabled in the Options page)
There are some options available, but basically, the extension does just that and nothing more.