If no horses are around, with a water hose! It's super effective
I take a long stick and swirl it around the room
If the ceiling is pretty high, and water proof, you could use a pressure washer
One thing we have found is taking a broomstick and adhering a paint roller stick and pad at the end and roll it along the ceiling and sides of my barn then discard the roller pad. The roller pads can be purchased in packages of 5 for $1.50. Hope that helps!