vlock is a program to lock one or more sessions on the Linux console.

This is especially useful for Linux machines which have multiple users with
access to the console. One user may lock his or her session(s) while still
allowing other users to use the system on other virtual consoles.

If desired, the entire console may be locked and virtual console switching
disabled.

On machines that support "Magic SysRq" (enabled via a compile-time option
and /etc/sysctl.conf setting) users that lock all VT's on the machine must 
be members of the "wheel" group.  Note that 12.0's stock kernels include 
support for the SysRq key.