internet connection sharing

I have enabled internet access on me windows 2000 server for my all workstation by internet connection sharing options in dialup connection.

How can I block internet access for unwanted IP like I don?t want to give access to all my LAN users I just want to give Access to some of my assign IPs.

I try to find something to manage that but I couldn?t find any thing and in reaction of that my whole staff is using inter net freely.

This is Adrehman
From ZRG international
Karachi, Pakistan