RewriteEngine on
RewriteCond %{HTTP_REFERER} !^http://yoursite.com/.*$      [NC]
RewriteCond %{HTTP_REFERER} !^http://yoursite.com$      [NC]
RewriteCond %{HTTP_REFERER} !^http://www.yoursite.com/.*$      [NC]
RewriteCond %{HTTP_REFERER} !^http://www.yoursite.com$      [NC]
RewriteRule .*\.(txt)$ http://www.yoursite.com/YMMV/index.php [NC,R,L]