GNU / Linux in general is great, it offers multiple options and we can modify it as much as our knowledge / ability reaches us.
Everyone knows what if in one TTY (terminal of [Ctrl] + [Alt] + [F1-F6]) we push [Ctrl] + [Alt] + [Del] two or more times, the system reboots. Well, this in my view is a huge error or failure, because suppose I have my laptop locked (because I left the office), any stranger can come and without the need to enter username and password, I can restart the computer.
I just discovered that this can be solved, just by commenting (or deleting) a line in a ready file, our computer will not be able to restart with [Ctrl]+[Alt]+[Del] 
The file that we must edit is / etc / inittab and, to edit it:
- Press [Alt] + [F2] and in the box write «gksu gedit / etc / inittab« (without the quotes)
- The previous step will ask us for a password, we put it and press [enter] once again.
- A text editor will open, let's go to the line 40 which should have the following: ca :: ctrlaltdel: / sbin / shutdown -t3 -r now
- We must leave it with a number symbol in front (#), being that way: #ca :: ctrlaltdel: / sbin / shutdown -t3 -r now
- We save and close the file.
- Now let's open a terminal (console, bash, shell, whatever you want to call it)
- In this let's say: sudo init q and press [enter]
- This will help the system re-read the file that we previously edited.
Ready, our system will not restart if in a terminal we press [Ctrl] + [Alt] + [Del]
regards