Limiting Access to Users and Pages

To limit access to a specific node or type of node, use the Content Access module (http://drupal.org/project/content_access).

To limit users from editing their user information (changing password, username, e-mail addy, etc.), use the User Protect (http://drupal.org/project/userprotect).

To redirect users to a login page instead of an Access Denied page:

Go to Administer > Site configuration > Error reporting and providing the drupal path for the 403 page: user for login page