DHCP

Let’s see if I can make this sound right.

I have a specific domain, however, I have contractors come into our office. When they do, the can connect to our LAN drops and our DHCP assigns them an IP address.

What I want to do is to ensure that any machine that does not belong to my domain will not get an IP assigned to them and they will get no access to the internet.

I have thought of static IP’s howeve they will fix the problem however, I want to be able to manage my IP range at a central location.

Please any help would be much appreciated.
Thanks in advance for any help you might be able to provide.