If you install dbmail, you will see the "user" account that it creates in your
login screen.  It has a shell of /sbin/nologin so it will fail to login. 
Therefore the gui should not show this account.  dbmail is using fedora-useradd
to add the account and ends up with a UID > 500.  I am afraid just checking if
the account is < 500 is not enough.  You might have to hard code, checking for
the shells /sbin/nologin and /bin/false.