How do you secure DHCP and regulate which pc's get addresses?

Example: We have many contractors that bring their own personal
laptops
to work and plug into the network. Since we use DHCP they receive an
IP
address and are thus able to see all network devices.

Is there a way to limit or regulate which machines can get an IP
address? I don't want to use reservations since we have over 400
pc's.

NetWare 6sp2, Clients will all soon be XP.

Thanks,
Crystal