First you need to be aware the many nic card drivers do
allow one to set the MAC address that will be used.
You first ask about controlling access to a W2k server,
and next about controlling leasing of IPs from DHCP.
Are these two different concerns or a reexpression of
one? If one, then DHCP will offer an IP to any machine
that asks, if it has available IPs. You can reserve IPs
that are then available each only for a specific MAC,
and if all IPs in the scope are so reserved then there
are none available to MAC addresses which have not
been so provided for. This is sort of a tedious way to
get at what you are after, except in reverse.