Yes you can change access to an application or a page using the Permissions settings. For content pages there's a Permissions tab you can go to for each page.

For content pages this can only be done with top level sub-pages. Any pages you create as sub-pages of sub-pages will take their permission from the top sub-page. (if that makes any sense!)

In the standard site setup, Read permission is granted to The Public and is inherited down to every item on your site. To remove this from a page/application then you can go to the appropriate permission tab, click the "Do not inherit permissions..." button, then alter the permissions explicitly for that page.

Thanks

Joe