Chili powder is an effective dog repellent when sprinkled around the property. Chili peppers contain capsicum, which irritates the dog's skin, especially near the nose.
The scent of ammonia is extremely unpleasant to dogs, so leaving cotton balls soaked in ammonia around forbidden areas can be an effective deterrent. Ammonia kills plants, so avoid planted areas. Vinegar and rubbing alcohol can also be used in the place of ammonia.
Citrus oil or the peels from grapefruits, oranges, lemons or limes can repel dogs. Citrus is an appropriate alternative when substances with harsh odors are not appropriate. There are commercial dog repellents that are usually citrus scented, which is ideal for when citrus peels themselves would be an eyesore. There are also dog repellents that are scented with black pepper, according to Gardening Know How.
Products are available in the marketplace that can clear an area of familiar smells. Dogs often return to the same spots to do their business, and masking familiar scents can keep them away. Another option is to use a device that emits ultrasonic sound waves, which humans cannot hear. It can be mounted outdoors and set to stay on consistently or to activate when motion is detected.