It depends on what type of occluder that your are trying to edit. Some, like terrain, doors and pits need to be closed polygons, so if you delete a point it will automatically join itself together to preserve its closed status. There isn't an automatic way to change from one occluder type to another, so you'd need to delete the first and then re-draw it as the one you want.
If you want to make a gap in a wall (or other occluder type that isn't a closed polygon) then you can draw a square on top of the wall line and then delete the square which will leave two points on the line which you can use to delete a portion of the wall. You can also draw a small wall section bisecting a line and use the same trick to remove a section. See the videos on LoS in the first post for examples of how to edit occluders that have already been drawn.