Another idea might be to spray the nest with your hose every time you're out there. Got a good nozzle that really shoots a long stream? Really wet it down and tear it up if you can with the water. The birds will at least move the nest but the water most likely will not hurt anyone.
Any chance you can prune the tree/shrub they are nesting in so it's not so comfortable for them? If you can make them uncomfortable enough, they will surely go elsewhere.
