If the journal is configured to archive entires these can take up unnecessary disk space when not properly maintained. The current disk usage can be seen with journalctl --disk-usage. This includes both active and archived journal files.
Clear journal entries older then X number of days
$ journalctl --vacuum-time=5d
Clear journals exceeding X storage
$ journalctl --vacuum-size=2G
Flush journal to disk
Vacuuming only impacts journals that have been written to disk. It may be necessary to first flush in-memory journal entires to disk before a vacuum reduces disk usage.
$ journalctl --rotate