I've parsed my server logs and altho it accounts for 1% of the traffic I still believe its something that should be prevented as soon as possible.
Among the solutions proposed here I think another one is to inform the user.
Preview Here
If you want to inform your surfers you can create a landing page like that or use mine and add the following .htaccess code to your sites:
Code:
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} ^.*ZangoToolbar.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://www.dark5.com.*$ [NC]
RewriteCond %{HTTP_REFERER} !^http://www.yourdomain.com.*$ [NC]
RewriteRule .* http://www.dark5.com/warnings/zango.php?ref=www.yourdomain.com [R,L]
I've added it to a couple of my sites and will add it to all.
If all big traffic sites would use something like this we could accomplish something.