If seeing the update message at the bottom of the window poses a risk to your sanity

you can try disabling it by entering the Calibre web site address (
www.calibre-ebook.com) in your browser's restricted sites list. If using IE it is at Tools>Security>Restricted Sites>Sites Button (that in IE10, from memory earlier versions are similar).
Of course, you will have to re-enable it if you want to go to the site for any other reason, or else use a second browser.
EDIT: If that doesn't work try adding
127.0.0.1 calibre-ebook.com to your hosts file