To safely and effectively knock down a wasp nest, it is recommended to do so at night when the wasps are less active. Wear protective clothing, such as long sleeves, pants, gloves, and a face mask. Use a commercial wasp spray or soapy water to spray the nest from a safe distance. Wait until all the wasps are dead before attempting to remove the nest. It is important to exercise caution and consider hiring a professional if the nest is large or difficult to reach.