Block direct url enter

Preventing users from entering URLs manually in the address bar is not possible in WordPress, or from any site, for that matter. It can only be done via a browser plugin.

However, redirecting users or displaying a 404 page for a certain URL is easy to do and there are several WordPress plugins for blocking admin access. One example:

http://wordpress.org/extend/plugins/wp-admin-block/

error code: 523