From the PDF manual: The sparse point cloud represents the results of image alignment and will not be directly used in further processing...
Delete sparse points make sense only using gradual selection function(from Model menu), which will results in better(more precise) camera positions. After selecting and deleting points using gradual selection tool, you need to use optimize cameras function from Tools menu to update camera positions.
If you want to clean dense cloud points, you can select them by color, by Mask, or filter them by confidence.(Tools-Dense cloud menu). Or you can export dense cloud to Cloud Compare program and apply SOR function which will remove points representing noise in cloud.