This is the second such nest I have on two edges of the same outside wall on adjacent years. I'm either inattentive, or they are particularly fast: the nest is built before I notice. One idea that worked with the first one was to spray shaving foam over it. Luckily the foam stayed there long enough, and the entrance was no longer menacing. The second one, pictured, seems to have served its function and is now empty.
The wikipedia page linked above suggests they might reuse a previous nest, and so I'll pull this off soon to avoid a recurrence.
Apparently they find something particularly attractive at this location. The color of the bricks? That the spot is shielded from rain? Is there a way to dissuade them from building in this particular spot?
I have also had yet another wasp nest below a layer of leaves in a bag near this entrance. Is this the same species? Might the presence of leaves act as an incentive to build the "organ pipe" mud dauber?