Any user can restart

[Catalin David]

Hello!

Well, my problem is not so easy to understand... but i'll give it a try.

Imagine this: you work on something, you don't save, take a break, you 
lock my screen, someone else comes to the computer, tries to login, 
can't guess your password and decides to switch user, this taking him to 
the login screen. There is a magic button on the low-left corner 
allowing the user to restart, shutdown without any need of password or 
so. You lose all your work.

Is there any way to disable this functionality (forcing a user to login) 
or to run it with gksudo instead (i mean to ask for root password)?

Thank you,
Catalin