Manage Everyone’s Reports and Dashboards

Use the allPrivate query scope to find and delete reports and dashboards saved in private folders. Requires the user permission "Manage All Private Reports and Dashboards."
Over time, reports and dashboards fall out of use, or users become inactive and leave behind obsolete reports and dashboards in their private folders. Previously, there was no convenient way to delete reports and dashboards saved in users’ private folders. Now, log in to Workbench and run a query to return reports saved in users’ private folders which haven’t been run for more than a year:
SELECT Id FROM Report USING SCOPE allPrivate WHERE LastRunDate < LAST_N_DAYS:365
You can also query reports saved inside a specific user's private folder:
SELECT Id FROM Report USING SCOPE allPrivate WHERE OwnerId = '005A0000000Bc2deFG'

Export the returned list of Report Id’s to a CSV file, then click data | Delete in workbench and upload your CSV file. Click Map Fields to map the Id Field to the CSV Field of the same name. Leave Permanently hard delete records and Process records asynchronously via Bulk API deselected, then click Confirm Delete. The reports listed in your CSV are moved into your Salesforce Recycle Bin.