vlock — Lock the Virtual console session.
vlock is a program to lock one or more sessions on the text console. This is especially useful for Linux machines which have multiple users with access to the console. One user may lock his/her session(s) while still allowing other users to use the system on other consoles. If desired, the entire console may be locked and console switching disabled. To unlock, Password of the user, who locked it, Or root is needed.
$ vlock — Lock the current session.
$ vlock -c — Same as above.
$ vlock -a — Lock all console sessions and disable the console switching.
Read: man vlock