Blocking particular website for specific thinclients on Ubuntu

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http://www.unix.com – We have server which is connected with more than 10 thin client machine. I have tried to block the websites ( facebook, orkut,twitter ) for all the users. it works fine for me. But , I want to block for particular user ( thinclient ) or by the way of IP address of machine. How can I do that. (HowTos)