How to delete Report or Dashboard from Private or Personal folder in salesforce?

Now Administrators can delete a private or a personal folder reports or Dashboard in either Workbench or Data Loader.

Before administrators delete private reports and folders

1. Follow the steps in Turn On Enhanced Sharing for Reports and Dashboards.
2. Export Report IDs to a CSV file which contains only the IDs of Reports to be deleted.
3. Decide if you will use Workbench or the Data Loader to delete reports.

Delete private reports and folders in Workbench

1. Login to Workbench.
2. Click the Data menu, then select Delete.
3. Select From File.
4. Select your CSV file, then click Next. 
5. Verify that ID is mapped correctly, then click Map Fields.
6. Map the ID field to the CSV column containing the report’s IDs.
7. Click Confirm Delete, leaving Permanently hard delete records and Process records asynchronously via Bulk API deselected.

Delete private reports and folders in Data Loader

Note: When running the delete operation in Data Loader, the ‘Report’ object is not visible.  The steps to do this in Data Loader should be as follows:

1. Select Delete then log in.
2. Select any object from the ‘Step 2: Select data objects’ list. (Please note, the Report object will not be visible. Data Loader will only check the ID, and will not check which object this is running against, so it doesn’t matter which object is selected.)
3. Click Browse then select your CSV file.  Map the ID field.
4. When prompted, click OK if the number of records to be deleted is correct.
5. Click Create or Edit a Map.
6. Drag the ID field down to the ID row then click OK.
7. Click Next then select Browse to select the location of your success and error files.
8. Click Finish.
9. Click Yes at each prompt to proceed with the delete operation.