Problem

You have started a couple of guest linux sessions on your pc, that you do not need anymore (ie giving access to your children while on holiday 🙂 ), and you want to close them from your own session.

Solution

Find out about the guest users by using who:

who
...
guest-kk9tnS :2           2015-08-09 17:39 (:2)
guest-kk9tnS pts/21       2015-08-09 17:40 (:2)

and then kill the sessions using pkill:

sudo pkill -9 -u guest-kk9tnS