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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