Block Internet Accesss to users not in my domain

Dear all,

We have a DHCP LAN of 100 PCs and Macs computers managed by a Windows Small Business 2003 Server and our problem is that users who are not members of our domain can have Internet connexion once they plugin a network cable to our network. My question is to know if there is somebody who acn help or advice me on how to can restrict Internet access to only our domain users.

Thanks for your suggestions.

Marcel