This OMERO command-line plugin allows you to compute the space per user, find the users with the biggest amount of data and free disk space on a server
To generate the list of users which data must be deleted to free 300GB on the system without running the deletion:
$ omero demo-cleanup --gigabytes 300
To generate the list of users which data must be deleted to free 300GB on the system and running the deletion (WARNING: data belonging to these users will be removed permanently):
$ omero demo-cleanup --gigabytes 300 --force