Win9x logon to Win2K server

I manage a network with a Win2K server and Win9x workstations consisting of desktops and notebooks.I have just discovered much to my amazement that one of my users using a personal notebook running Win98 has been connecting to my network and accessing the server without logging into the domain and using only share level security. All other workstations logon to the domain and uses user level security. He also uses a different workgroup name and yet he is able to access all files in the server according to the given user rights. Is there anyway I could stop such logons or connections to the network without upgrading all the workstations to run on NT platforms? Could someone offer some suggestions to control such logons?