To delete a report, click Dashboard to view your list of previously run reports.
Towards the top right, click Edit List:
You will then see checkboxes next to each report, and a box at the very top to select all. Check the box next to each report you wish to delete.
This will remove the report from your list of previously run reports. Please keep in mind, once you delete a report, it is not possible to "un-do" the deletion. You would need to re-run the search, in order to view the report again.
Please note - if you attempt to delete a report you are currently monitoring, you will be notified that you are trying to delete a monitored report, and will no longer receive updates for that report.