DH and family are die hard bird watchers. DH said that his dad just splashes some hot water into their bird bath, if there's algae growing in it. Just wait until the water goes down, scoop out the algae, splash with hot water, job done. I imagine it's the same for the poop situation, just over flow it with hot water so that it goes into the grass. But, yeah, I'd say just leave them to it. It's multi-purpose for them. Bathing, drinking, socialising. They probably think, "Hey cool, mini pond! I need a rest, let's go." Bees also drink from the bird bath when there aren't any birds around. I wouldn't really bother disturbing a bird bath much, myself. Otherwise, they won't come around.