You just now upgraded to Panther? From what? In the previous system, did you have a list of users that you could click on in the login box, or did you have a prompt to type in the user name?
I'm not sure there's a way to selectively hide users in the list mode.
One option is to turn off the list mode and force the user to type in a username. You can do this via System Preferences -> Accounts -> Login Options -> Display Login Window as Name and Password.
If what you want is to *delete* that user, highlight them in the Accounts pane and then click the (-) at the bottom of the column of users.