One way is to use the Starting Alias Path property of the User.

From the docs:

The alias path of the site section that opens in CMS Desk. If you specify this value, the user is not allowed to browse other sections of the web site in the content tree. Please note that this feature is only intended for better usability and it doesn't ensure security control - if you need to establish access rights for the given user, grant him with appropriate document permissions (Properties -> Security).

Doc page

Hello,

I just add to Elijah's propose, that you can set document level permissions for particular user-role to achieve this aim. You can inherit permissions on subpages to make things easier. Please see more details here: http://devnet.kentico.com/docs/devguide/document_level_permissions.htm

Best regards
Ondrej Vasil