Hide users from Mac OS X Snow Leopard logon screen

The easiest method for hiding system users (if their user ID is < 500) in the login window is to run the following command:

sudo defaults write /Library/Preferences/com.apple.loginwindow Hide500Users -bool TRUE

Alternatively you can manually hide just the username by running

sudo defaults write /Library/Preferences/com.apple.loginwindow HiddenUsersList -array-add '_postgres'

To hide the 'Others...' item from the login window if need be:

sudo defaults write /Library/Preferences/com.apple.loginwindow SHOWOTHERUSERS_MANAGED -bool FALSE

dscl . create /Users/test
dscl . create /Users/test UniqueID 420
dscl . create /Users/test PrimaryGroupID 420
dscl . create /Users/test UserShell /bin/bash
dscl . create /Users/test NFSHomeDirectory /tmp
dscl . create /Users/test RealName Test
dscl . create /Users/test Password test

This creates a user that's visible in sysprefs/Accounts.

dscl . create /Users/test Password "*"

This hides the user. Make sure you quote the "*" or it won't work.

EDIT: I accidentally managed to recreate googletorp's situation of not being able to hide a user by setting his password to "*", and I discovered how to fix it. This time, I had created a user using dsimport, like this:

dsimport /dev/fd/0 /Local/Default I --template StandardUser << EOF
test:*:520:520:Test user:/Users/test:/bin/bash
EOF

But in that command, the * is taken to represent a literal one-character password of *, and so dsimport creates an AuthenticationAuthority property for the user and sets the password property to the shadow hash of * (which shows up as ******** in dscl, as for all passwords). After that, attempting to set the password to "*" using dscl just keeps setting the password to a literal *, instead of disabling the password. The solution is to delete the unwanted property, and then disable the password:

sudo dscl . delete /Users/test AuthenticationAuthority
sudo dscl . create /Users/test Password "*"

This hides the user.

Just in case you haven't found a viable solution (or in case someone else finds this question from Google), setting the user's shell to /usr/bin/false prevents him from logging in and hides it from the login screen and from the system preferences. To do so, use the following command line:

sudo dscl . -change /Users/[username] UserShell /bin/bash /usr/bin/false

And to revert the change:

sudo dscl . -change /Users/[username] UserShell /usr/bin/false /bin/bash

Where [username] is the name of the user you want to hide (_postgres in your case I assume). I don't know why dscl wants the old value first, but that's what the manpage says, and it works quite well.