A device requests a page from the proxy, the proxy decides whether it has to fetch it, or can return it from the cache. So it has know what it has. Blocking is a simple step after that. Most proxy servers come with various tools for doing that.
Unless you are planning on banning them by url, no more you tube ever boys.. Not to mention telling a say the video for a modern dance tune and a porn film apart would beyond any automatic blocking system I've ever heard of...
Course stopping boys being boys is a different thing all together, perhaps you could start with something more achievable like cold fusion, or curing world hunger?
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