In order to do this, you have several choices:
1) Place the page inside of a 'Private' Folder so any user will need permission to login to view it.
2) Place the page in a Folder marked as a "archive".
3) Rename the page so that any link pointing to the page is broken.
If you want to modify a page that is currently viewable by the public and release the modifications at a later time, you will need to duplicate the page and make your changes to the duplicate. When you are ready to launch it, you will need to rename or delete the original page and then rename the duplicated page to take its place.