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