How do you keep birds from building nests on lights under soffits?

Living out in the country, we have pairs of spot lights located under the soffits on one side of the house so we can light up the yard if we need to. Swallows find the lights irresistible as a spot for building messy mud nests and we battle them for months every year trying to stop them. We've hung sparkly garland near the lights and that scared them away for a few weeks and now they are back. We are frustrated from the constant bird droppings and mud down the sides of the house but really like and need the lights. What works to keep them away?