I'd get with the BO and post some rules for everyone to see. Especially about letting you or whoever else is in charge get that brat off the horse before she kills it!
Maybe just a photo wall of poo-flinger of the month. Make the poo fun and seem like an honor to have done the best job or maybe most consistantly clean stall... Rewards could be as simple as a big candy bar or cupcake.
Be strong and courageous, and do the work. Don't be afraid or discouraged by the size of the task, for the Lord God, my God, is with you. He will not fail you or forsake you.
1 Chronicles 28:20