C3rd

There's probably an easier way, but I do this: See who's logged into your machine -- use who -u:

root@alphaone:~# who -u
root     pts/1        2016-12-08 11:02   .          7953 (192.168.0.99)
camilord pts/2        2016-12-08 10:59   .          7625 (192.168.0.7)

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

root@alphaone:~# echo "HAHHAHAHAHAHA... BYE!" | write root pts/1
write: write: you have write permission turned off.

Kill the corresponding process:

root@alphaone:~# kill -9 7953

  Reference: http://unix.stackexchange.com/questions/615/how-do-you-kick-a-benign-user-off-your-system