Block Internet Access to client system from Server

Hi Good Eve 

 Im Amul Raj . Im using windows server 2003 ,also i have 50 client systems , noe i block internet access to client system using servsr configuration , like as a set password, Every client system are need type password afterthat oly can access the internet , so i know how to block the internet pls help me .

Thanks for Advance

With Regards '

Amulraj S