I wanted to disable a user from logging in or appearing in the user list, but I didn't want to delete the account. It turns out this is possible using the "*" in the password field of the NetInfo database. Just like in Unix. Standard warning: Netinfo changes can render your system unusable ... so use caution!
Using NetInfo Manager, select the user's short name and enable NetInfo changes by clicking the lock icon. Select the user's password entry and replace the encrypted password with * (shift-8). Save your changes. The user is now disabled and cannot log in. Even better, they won't appear if you have "List of Users" selected for the login screen. This is useful to lock an account that might be compromised or hibernate accounts not needed. To re-enable the user, just reset the user's password in the Accounts Preferences.
NOTE: DO NOT disable the administrative account (the first account created). I haven't tried it but I'm sure it would be very, very bad.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030429184312327