You could create a listing of the top 20 or so 'Abusing Space Teachers' and see what peer pressure would do ... if you are very secure in your job ... or don't care about your job! :|
Could pay them a stipend ... for cleaning private files backups off server ... $ talks, but that is also too far fetched ... then again ...
Don't think you, even as an admin level user, can see Teachers private files - that's where teacher made backups would go.
That's one reason the plugin you are using is perhaps best ... even if you have a lot of clicking to do ... cause that plugin can see and manage those teacher private files.
There is another way ... would involve not only command line scripting and *very careful* manual or loop scripted deletions of files in /moodledata/filedir/xx/yy/xxyybunchletersnumbers ...
There is a moosh command:
https://moosh-online.com/commands/
file-delete
Delete Moodle files from DB and possibly move them to trash. File IDs can be provided as arguments or on the standard input (with moosh file-delete -s). --flush option will remove the trashcan directory.
Sorry ... no easy button! (former and still admin of a K12 moodle whose had to increase space 2 times ... now 2 Terabytes and at 60%.
Hmmmm ... maybe that's the other ... have moodledata on a mounted BIG BIG data server or data device - and just continue to grow it! :|
'SoS', Ken