If you install Postgres from enterprisedb.com then you’ll have a PostgreSQL user created that shows up in your login screen. It’s not the end of the world but is mildly irritating so here’s how to hide the account from the GUI:
sudo dscl . create /Users/postgres IsHidden 1
Note that /Users/postgres is not the home folder of the user but simply the path /Users/ followed by the user’s shortname. I tried pointing the tool to the users home folder with no luck.
Thanks to Apple Support for this answer. http://support.apple.com/en-us/HT203998