MacOS – skill -KILL -u $user Command on Max OS X

You could try using an Apple Script, something like;

tell application "System Events"
    log out
end tell

Then schedule it using launchd. I've used this as a nice GUI front-end to launchd in the past;

http://www.peterborgapps.com/lingon/

launchd can be a little frustrating to use as you have be very exact with your syntax and naming, however if you don't want to spring the $5 for Lingon, there is a Ruby Gem on GitHub I found called lunchy

https://github.com/mperham/lunchy

The downside with this is that you're running to a schedule rather than only running the script when a user has logged in.

sudo launchctl bootout gui/$(id -u <username>)
or
sudo launchctl bootout user/$(id -u <username>)

Replace username with the target user's user name or replace the whole subshell with the user's uid. This tells launchctl to teardown the users login session (gui specifically refers to the user's temporary login session, user specifies the users background processes).

You can log yourself out without the sudo to test this.

Note that this will ONLY work on macOS 10.11.x or newer (see launchctl help for more)