Michael's answer is missing one thing: vacuuming only removes archived journal files, not active ones. To get rid of everything, you need to rotate the files first so that recent entries are moved to inactive files.
So, the complete answer to remove all entries seems to be
journalctl --rotate
journalctl --vacuum-time=1s
(Note that you cannot combine this into one journalctl command.)
By the way, some distributions have journald configured so that it writes logs to disk (/var/log/journal) while others keep logs in memory (/run/log/journal). I expect that in some cases it may be necessary to use journalctl --flush first to get everything removed.