don’t want that people use you pc even in a secure way? just remove the guest account on your ubuntu and setup a password.
past the fallowing code
gksudo gedit /etc/lightdm/lightdm.conf
a note page will apear then past the fallowing code that the end
it sould look like this
[SeatDefaults] user-session=ubuntu greeter-session=unity-greeter allow-guest=false
then put this code in the command line (it will log out so save any other project that you are working on to do this)
sudo restart lightdm