Restricting Internet access on Linux 6.2

I have a network of Windows 95/98 pcs that have got to be connected to the e-mail service. I have a Linux server that acts as an e-mail/internet server and would like to restrict some users from having access to the internet but have access to e-mail. Some of the pcs are shared by a number of users so there are security concerns which need to be addressed. Could somebody out there please help