how to restrict internet access for users in windows 7

hi, I am using windows 7 32 bit . there are 90 users . we have linux server and run an web based application using IE. Internet is accessed through a modem which is connected to the same hub which uses the web based application. so everyone is having an access to the internet. How do i restrict the internet facility to only to 3 or 4 users. any guidance will be greatly appreciated