Limiting Access for non-domain Laptops

Hello,
I work in a company with about 100 users. We use windows server 2003, DHCP Server and ISA server to access to Internet. We don't have wireless access. Some people bring their laptops and connet it to terminal user and change their IP Address and use their username and password to use Internet.
My question is how can I limit the laptops to access to the network and Internet. I can reserve IPs to mac address but some people change the mac address by software.

Thanks,

Dont know if this will help you out any or what your knowledge consists of, in AD you can limit the PCs connected to your Domain, therefore if you add a new PC with a new name it wont be able to login to the domain and can restrict access. Somewhat new to this myself but just an observation.