Though this is trivial to circumvent, an easy way to block by User Agent with Apache is to use mod_rewrite.
This is rather more useful to deny spam bots, scammers, and other automated tools from poking at your website!
The following rewrite rule gives a 403 Forbidden for example:
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} HTTrack
RewriteRule ^.* - [F]
Place this in either your virutalhosts config file, or an .htaccess file.
PS: There is a well known but somewhat dated ‘ultimate’ htaccess blacklist available here which has a compiled list of bots to deny:
http://perishablepress.com/press/2007/10/15/ultimate-htaccess-blacklist-2-compressed-version/
Sweet!
