Restoring IP Address

As a "network admin" you should have the right to track this individual down and provide them with an alternative. So the question is "how to do this?"

Use the "arp" command to try and match up the required IP address with the rogue system's MAC address. If you have the documentation to match, then you can simply locate the MAC address on your list of systems. If you don't, then you'll have to track down the rogue in different ways. Depeding on a lot of variables, you might be able to PING the IP address and your system MIGHT give back a system name. Eee-Zee then. Otherwise, I would log into the LAN switch and try to locate the system's MAC address on that switch.

Failing those, then I would stay late or arrive real early. Set my PC up with the required IP address (assuming that the rogue system is shut down). When they start up somebody is going to get a "duplicate address" error.

In any case, you actually have a different problem. You either have a rogue user (imagine, setting up their own Network config!) or you have another network admin with whom you are not communicating. If the latter, then you've got to get this other admin talking to you.

Best o' Luck

Check WINS database. You can set it to show every computer name with the corresponding IP address. Unless you clear the database, it will be there since it is not leased. Since you have a record of who uses which computer name there shouldn't be anyproblem.
I hope this solves your problem.

Or you could tracert to the IP, this should give you the Netbios ID.. if you have configured your machine names in an appropriate manner.. such as geogrphic location then you'll know who it is..
OR block that IP from accessing your network resources. This way the user will have to request a new IP thus freeing up the one you need.