I was just looking for a possibility to lock a virtual terminal.
What I found is vlock. With vlock you can lock your terminal session very easily.
You first have to install vlock.
sudo apt-get install -y vlock
When you are in a terminal, just enter this to lock session:
Now you must enter your or the root's password to unlock. If the process is killed by someone else, the session is no more accessable.
I find this very useful, because many times I work on terminal and I don't want other people see what I'm doing.