To do so, open your wp-config.php file and paste the following code:
It will block external requests from that time on. Though, some plugins need external request to work properly. If you experience problems, you can define a whitelist by pasting the code below into wp-config.php. Don't forget to replace my url by the one needed by the plugin.
This is the last recipe of 2009. Thanks to all of you for your support, and best wishes for 2010!