Don't you keep a pointy stick behind your door? You could consider one of a few approaches.
1. Put a LGPO logon script on them that checks HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters, "DhcpDomain" (untested)
2. Sniff the DCHP requests going to the server cuz the machine will always try to renew the last IP it had.
3. If the laptops don't roam to other subnets, you could give them fixed IPs.
4. If they do roam but always use the same DNS, you could fix the DNS setting.
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Give a man a fish and he will be back for more. Slap him with a fish and he will go away forever.
