Produces an error 404 when accessing wp-login.php without a parameter.
###1. Add these lines to your .htaccess file.
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} wp-login.php
RewriteCond %{QUERY_STRING} !(access=yes)|(action=logout)
RewriteRule . - [R=404,L,NS]
</ifModule>
###2. Install and activate the plugin. ###3. Call your wp-login.php like this: http://example.com/wp-login.php?access=yes
Read more about this here (German language only): http://hinterland.florian-simeth.de/wp-login-php-umleiten-falls-kein-php-parameter-angegeben-wurde/