Disabled accounts still show up in login screen and session [$10]

[william ledoux]

When creating a user with the following command:
    $ sudo useradd -d /dev/null -s /bin/false -M foo

An account appears in the "Other Accounts" category and say Password: "Account disabled" which is fine*

The main problem is that this user (that can not log in) also appears in the login screen and in the session switcher of wingpanel (cf. attached screenshot)

* Except that if one create many accounts like this for administration purpose, he may want to hide them, or to have them in a "Disabled Accounts" category, below "Other Accounts"