If I delete users, will my storage usage decrease?
If you delete users, those users will be removed from your ticket metadata and will marginally decrease the amount of data stored in your instance.
Each user is counted as 0.05 KB of data, which means deleting users may only minimally decrease your overall storage.
To reduce usage effectively, use ticket deletion instead.
For more information, see this article: Managing data storage in your Zendesk account.