Lemon juice is one of the most effective fly repellents that you can use. The citrus smell will repel the flies. Simply add lemon juice to a spray bottle and spray it on the outside walls of your barn. To make sure that the flies don’t come back, you need to apply it every week. You can also use essential oils from herbs that are know as fly repellents. To make your own fly spray, simply mix equal parts of citronella oil and lemongrass oil. You can then spray it on the inside walls of your barn. The citronella and lemongrass oil together have a very powerful effect on flies. You can even combine essential oils with vinegar to make it more effective. However, if you are planning to use the repellent on horses directly, you should dilute it with water to avoid irritation of horse's skin.