I feel its impossible to block anyone from certain websites if they know how to get around the URL/DNS (ex..
www.myspace.com http://myspace.com) The majority of schools/houses/work cant imagine the amount of ways you can access a website without using their direct URL. For crying out lout just go to Google.com and type myspace.com proxies and there are literally hundreds of them you can use. Not to mention all the other Web Browsers you could download if you really wanted to. Always remember whoever made the security knows a way to break it.