I never knew such things existed until I read this article http://www.communitycare.co.uk/Articles/2009/09/18/112610/psychiatrists-call-for-crackdown-on-pro-eating-disorder-sites.html

We haven't needed to get to grips with parental control on the Internet yet so i don't know how it works, but assuming you can bar access to particular sites you may want to get to work on these.