I usually put this in my htaccess for my wordpress sites:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_USER_AGENT} .*Twice.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*Yand.* [NC,OR]
RewriteCond %{HTTP_USER_AGENT} .*Yahoo.* [NC,OR]
RewriteCond...