第一步：

run the command(s) below:        （编辑如下文件）

sudo vi /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf

Add the following line, save and then close.     （在文件中加上下面这句）

allow-guest=false

Reboot, and you're set.

 

第二步：

Thanks to

Based on Gunnar Hjalmarsson comment "ubuntu-session was last updated in the middle of March. Try sudo apt-get install --reinstall ubuntu-session" when that is done, the Guest session returns.      （第一步不是狠完善，guest可以通过某些命令恢复）

After some research, this can be done a different way as well.       （除了第一步外，下面这步也操作下）

Open /etc/lightdm/lightdm.conf file from your terminal using the following command:     

sudo -H vi /etc/lightdm/lightdm.conf

If the file exists, then just add the following line:

allow-guest=false

Otherwise copy and paste the following into it:    （如果文件不存在，则新建并加上如下的行）

[SeatDefaults]user-session=ubuntugreeter-session=unity-greeterallow-guest=false

Save the file and exit the editor. Reboot your system, and the changes will remain even after an update to ubuntu-session as mentioned by Gunnar Hjalmarsson's comment.