On any server, the logs can start to add up and take considerable amount of disk space. Systemd conveniently stores these in /var/log/journal and has a systemctl command to help clear them.
The first one is time-based, clearing everything older than say 10 days.
$ journalctl --vacuum-time=10d ... Vacuuming done, freed 2.3G of archived journals on disk.
Alternatively, you can limit its total size.
This example will keep 2GB worth of logs, clearing everything that exceeds this.
$ journalctl --vacuum-size=2G ... Vacuuming done, freed 720.0M of archived journals on disk.
Afterwards, your /var/log/journal should be much smaller.