Yes, although some browsers will ask permission from the user before actually closing. If the user says NO, the browser will remain open.
You could do that PLUS an alert message (if the user leaves it open) that says the window MUST be closed, or the site cannot perform nightly maintenance. Just a thought.
If anyone knows of a website that can offer ColdFusion help that isn't controlled by neurotic, pedantic jerks* (stackoverflow.com), please PM me with a link.
* The neurotic, pedantic jerks are not the owners; just the people who are in control of the "popularity contest".