Reduce size of /var/log/journal

$ journalctl --disk-usage
Journals take up 3.8G on disk.
You can control the size of this directory using this parameter in your /etc/systemd/journald.conf:

SystemMaxUse=50M

You can force a log rotation:

$ sudo systemctl kill --kill-who=main --signal=SIGUSR2 systemd-journald.service
NOTE: You might need to restart the logging service to force a log rotation, if the above signaling method does not do it. You can restart the service like so:

sudo systemctl restart systemd-journald.service