If you have a cisco switch in use at your location, you can run a command from the switch such as :
show mac address-table | incl 000F:1FC9:0F34
That should give you either the port its connected to, or the uplink. If its the uplink, you run the same command form the uplink'ed switch and check for the same result (btw you check for uplink with SHOW CDP NEI )
Once you have the port on the switch, you can trace the cable back to the desk of the offending system.
