To mass delete the lines in the display (Prospect, task, customer), open your admin area and click the menu where there are the lines to delete. At the top of the table, on the right, there is a checkbox to select all the lines in the table. You can just uncheck the lines you don't want to delete.
After selecting the lines to delete, you will find a "Bulk action" button among many others. Click on this button and a pop-up window will open. Check on "Mass delete" and Confirm.
