To delete a marker, press CTRL and click on the marker.
To delete one segment in a line, click the last point of the segment.
To delete the entire line, press CTRL and click any part of the line.
To delete one segment in a polygon, click the last point of the segment(for triangles & rectangles, segments cannot be removed).
To delete the entire polygon, press CTRL and click anywhere in the polygon area.
To delete the entire rectangle, press CTRL and click anywhere in the rectangle area.