Default Plone behavior is to display login box when the user tries to access content for which he or she does not have permission.
How to change this behavior so that instead and Unauthorized page is displayed? How to customize this unauthorized page to have custom texts. etc?
Customize this template: Products.CMFPlone/Products/CMFPlone/skins/plone_login/require_login.py, either via "old style" skin customization or "new style" jbot customization: http://pypi.python.org/pypi/z3c.jbot (see CMF Objects section)