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.

how?:

control+alt+t

past the fallowing code  

gksudo gedit /etc/lightdm/lightdm.conf

a note page will apear then past the fallowing code that the end

allow-guest=false

then
control+s

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