Select Delete
Vertices 
from the toolbar or the Edit
sub-menu of the Boundaries menu.
You can select individual vertices by clicking on them with the left mouse button, or you can select multiple vertices by drawing a window around them (click and hold the left mouse button, and drag to create a window).
When you are finished selecting, press Enter or right-click and select Done Selection.
All selected vertices will be deleted and all boundaries will be redrawn using the remaining vertices.