What kind of database are you running? If you’ve followed Zammad’s advice and you are running PostgreSQL, you can see where the majority of diskspace is being used. It’ll probably be in the article attachments. You could delete old tickets or spam via the Scheduler, check which ones have unusually big attachments and delete those tickets. You’ll also have to reclaim diskspace in PostgreSQL afterwards though.
Please check the PostgreSQL Wiki on Disk Usage for information on how to see which table contains the most data. It’ll likely be a pg_toast relation, containing the attachments.
For comparison, here is a TEST system, containing barely any data:
The following is from one of our PROD systems, with lots of articles, customers and attachments (the pg_toast relation at the top). We’ll probably need to prune this at some point, as the indices are getting a bit large:
You could migrate the attachments from the database to a directory on disk, for instance on a new disk or dedicated mountpoint. This has both advantages and disadvantages compared to storing the attachments in the database. Read this for more information on storage providers