This is answered here:

A copy of the steps:

There's probably an easier way, but I do this:

1. See who's logged into your machine -- use who or w:

    > who  
    mmrozek  tty1         Aug 17 10:03  
    mmrozek  pts/3        Aug 17 10:09 (:pts/2:S.0)
   

2. Look up the process ID of the shell their TTY is connected to:

    > ps t  
    PID   TTY      STAT   TIME COMMAND  
    30737 pts/3    Ss     0:00 zsh
   

3. Laugh at their impending disconnection (this step is optional, but encouraged)

    > echo "HAHAHAHAHAHAHAHA" | write mmrozek pts/3
   

4. Kill the corresponding process:

    > kill -9 30737
   

I just discovered you can combine steps 1 and 2 by giving who the -u flag; the PID is the number off to the right:

> who -u
mmrozek  tty1         Aug 17 10:03 09:01        9250
mmrozek  pts/18       Aug 17 10:09 01:46       19467 (:pts/2:S.0)