Block access to network

DEAR COLLEAGUES,
I work in a Windows 2000 environment with Win2k pro and xp clients OS. I have fully implemented Group Policies. However some of the users of laptops have decided not to have their laptops added to the domain. SO WHAT THEY DO IS FIND ANY PATCHED CAT5 PORT, PLUG THEIR LAPTOPS IN, OBTAIN AN IP ADDRESS AND OFF THEY GO. IS THERE A WAY I COULD DENY THEIR LAPTOPS IP ADDRESS OR BETTER STILL STOP THEM FROM ACCESSING NETWORK RESOURCES UNLESS THEY ARE PROPERLY LOGGED ON TO THE NETWORK.