Such feature is not yet implemented in the CMS and I have logged it PITS
so you can track the progress and vote for the item.
As a workaround I can suggest you to put a app_offline.htm in the root of your application. The file will turn off the site and will display the contents of the htm file instead. You can also set a second site ( accessed only internally ) using the same copy of the database and edit the internal site.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items