Phorm = Lying Bastards
Here's what they say for web site owners to preclude their web site traffic from being intercepted. They basically say you have to (1) encrypt your traffic, (2) require all visitors to log in via RFC 1945, or (3) use a robots.txt file to block *ALL* web crawlers. In other words, they are claiming the only way to transparently protect your visitors from their criminal behavior is to block *ALL* search engines of any kind from crawling & indexing your site.
http://www2.bt.com/static/i/btretail/webwise/help.html#how-do-i-prevent-webwise-from-scanning-my-site