You cannot delete more than one set of results at a time from Silk Performer's Workbench as you can only select one result set at a time.  A change request has been logged to be able to delete more than one set of results at a time.

You delete the results manually from the projects Folder using Windows Explorer or My Computer.  After manually deleting select 'Local Results' in the Silk Performer Results Project window > right click and select ' Update local results'.  This will then update the results in Silk Performer and remove any which have been deleted.