That doesn't stop them from changing the IP. They just have to change the client settings from dhcp to static ip.
1) When u load any OS do u allow the user permissions to change the ip or mac address if so then the setup and policy is bad. Admin user rights are usually kept with the owner or admin.
2) Also if u use squid and if he does change the ip would he not have to know ur network ips range 2 do that. So if the IP is out of range in squid would he still be able to log on to the net.
Derwyn