Turnitin.com offers various services to the educational community. Most prominently, we provide a widely used and effective plagiarism detection service. Part of the plagiarism prevention service relies on comparing student papers to content found on the Internet. Since we do not know ahead of time which pages on the Internet a student will use we need to gather them all for comparison. However, we do have automated ways of throwing away content and links that would be irrelevant to our service.
Add this to your site’s /robots.txt:
User-agent: TurnitinBot Disallow: /