Here are a couple of recipes that work well. Using a spray bottle, add 2 tsp. citronella oil, 2 tsp. eucalyptus oil, 2 tsp. dish detergent, 1 cup oil and 2 cups water. Shake well and spray the area. Another is 1/2 Cup white vinegar, 2 Tblspns dish detergent, 2 Cups water, mix and spray. Or fill a jar with 3-4 Cups of water, 1/4 Cup white vinegar, 1/4 Cup sugar. Punch a large hole in the lid of the jar and place where flies are most annoying. Keeping the stable as clean as possible will also help.. Use green products that won't hurt your horse to clean.