Teach them to edit
/etc/systemd/journald.confand you teach them how to solve the problem permanently.
More specifically: Activate the SystemMaxUse= option there, e.g. as SystemMaxUse=100M to only use 100 MB.
After editing, use service systemd-journald restart to activate the changed configuration. This will remove the excess logs.
journald.confalso has other options that might be useful.