How to control the Internet usage

Corrently my company have assigned a range of ip from x.x.208.x that can surf the web(set this range to bypass from the firewall); most people in our company uses DHCP to obtain ip(range excluding x.x.208.x), other that knows(given the rights) theseip addr range uses statics ip(can surf web). Now the problem is that when other people that dont have the rights to surf the web obtain this ip addr range they can surf the web(without our approval).
How do I overcome this proble? Thanks.