2 methods:
A) First so you don't have to manually go through each one, make sure their in their own COS (alumni or graduated etc)
(you can even script the COS change if you have a list of the user's handy)
In the admin console gui > cos (or individual user) > advanced tab > scroll to the bottom > and you can set:
Email message lifetime -Number of days a message can remain in any folder before it is automatically purged. The default is 0; email messages are not deleted.
Set this to 1
(let me know if you want zmprov commands instead-but it's really easy to find)
B) There's also the emptyFolder command but if they have /Folder at the same level as /Inbox it doesn't help you at all.
zmmailbox -z -m
user@domain.com emptyFolder /Inbox
OR
zmmailbox -a admin -p password -m
user@domain.com emptyFolder /Inbox
OR
zmmailbox
mbox> adminAuthenticate -u
https://server.domain.com:7071 admin@domain.com password
mbox> selectMailbox
user@domain.com
mbox
user@domain.com> emptyFolder /Inbox
I would do A because it's the easiest, followed by B for the /Calendars /Notebook etc
You could get pickier on folder names/write a script to get all folders etc, but that's probably most of the used storage space anyways.