When a client, on a different subnet than the Windows 2000 DHCP Server, sends a DHCP release message, it may be ignored?
This problem can occur if the DHCP Server does not have a scope defined for its' local interface.
To fix this issue, create a scope on the same IP subnet as the DHCP server's interface. The scope does NOT have to be active.
NOTE: You can manually delete the leases by using the the DHCP MMC.