1. Never allow any Temporary User without a password to gain Access Control of your computer! FIRST Make a policy to prevent the single user guest from making system wide changes open text editor gksu gedit /etc/polkit-1/localauthority/50-local.d/10-desktop-policy.pkla insert text

[guest-policy]
Identity=unix-user:guest
Action=*
ResultAny=no
ResultInactive=no
ResultActive=no

2. open terminal and start typing

sudo addgroup --system --quiet --gid 126 guest
sudo useradd -c Guest,,, -d /tmp/guest-home.UBUNTU -m -s /bin/bash -g guest guest
sudo usermod --uid 117 --gid 126 guest

to create blank password for this account:

sudo usermod --password U6aMy0wojraho guest

to create Not asked for password on login for this account:

sudo usermod --groups nopasswdlogin guest

Edit /etc/gdm/gdm.schemas type:

gksu gedit /etc/gdm/gdm.schemas

and add guest to greeter/Include default

    <schema>
      <key>greeter/Include</key>
      <signature>s</signature>
      <default>guest</default>
    </schema>

now sudo restart gdm

NOTE: you will no longer be abel login to guest sessions from user accounts this is the new guest session and you will only be able to login from login screen any changes to this account will remain on logout until the computer restarts.

to restore open terminal and type:

sudo userdel guest

then sudo restart gdm

to copy your settings for guest session

sudo cp -R ~/.gconf/desktop /etc/gconf/gconf.xml.system/
sudo cp -R ~/.gconf/apps /etc/gconf/gconf.xml.system/
sudo chmod 777 -R /etc/gconf/gconf.xml.system/desktop
sudo chmod 777 -R /etc/gconf/gconf.xml.system/apps

Updated: 2016-Aug

For Ubuntu 16.04 LTS (15.10 or later)

Adapted from: LightDM Ubuntu Wiki - Configuration

• System provided configuration files in /usr/share/lightdm/ are not intended to be user editable. Instead users should use: /etc/lightdm/
• Newer versions of LightDM (Ubuntu 15.10, or later) use [Seat:*] instead of [SeatDefaults]

Just run this (once) at terminal:

sudo sh -c 'printf "[Seat:*]\nallow-guest=false\n" >/etc/lightdm/lightdm.conf.d/50-no-guest.conf'

To undo (restore Guest option), remove the file created:

sudo rm /etc/lightdm/lightdm.conf.d/50-no-guest.conf

For Ubuntu 14.04 LTS (up to 15.04)

Adapted from: Ubuntu Forums - lightdm config files location in 14.04

Just run this (once) at terminal:

sudo sh -c 'printf "[SeatDefaults]\nallow-guest=false\n" >/usr/share/lightdm/lightdm.conf.d/50-no-guest.conf'

You will no longer have 'Guest' as login option, on your next login.

To undo (restore Guest option), remove the file created:

sudo rm /usr/share/lightdm/lightdm.conf.d/50-no-guest.conf

For Ubuntu 12.04 LTS (up to 13.10)

Simplest method.

Just run this (once) at terminal:

sudo /usr/lib/lightdm/lightdm-set-defaults -l false

You will no longer have 'Guest' as login option, on your next login.

• This simply appends allow-guest=false to /etc/lightdm/lightdm.conf.

• Should work from 11.10 onwards (since these use LightDM as well).

To undo (restore Guest option):

sudo /usr/lib/lightdm/lightdm-set-defaults -l true