Vlock

vlock is a Virtual Console lock program.

Concepts
Sometimes a malicious local user could cause more problems than a sophisticated remote one. vlock is a program that locks one or more sessions on the Linux console to prevent attackers from gaining physical access to the machine.

Emerge
To install :

Usage
When not working in a virtual console, switch to one by pressing ++ through. By default, locks the current console session. Use the  switch in order to lock all console sessions.

It is also possible to use vlock from an X session. Use the  option to make vlock switch to an empty virtual console.

Disable SysRq key
The magic key combination can unlock consoles when least expected. In order to prevent this, disable the SysRq mechanism while consoles are locked like so:

If a user does not know how to use the key, then it is probably not needed. Disable it when configuring the kernel: