Is there any way you can track down the MAC address of the offending computer? If you can get the MAC, the you may be able to track it to a specific network jack in your building.

If you use Cisco switches, and the offending computer is still on the network, then you can track it down to a specific port with the show mac-address-table address xxxx.xxxx.xxxx command. You'd have to run this command on every switch until you found a match.