So that the smell of the female dog that is in the estrus cycle won't be perceptible to the male dog, currently in the market there are canine physiological repellents, for oral or external use, which create a smoke curtain.
To definitively avoid the possibility of the female dog getting pregnant, there is the option of a surgical intervention, obstructing the ovary conducts. Some veterinarians say that the intervention can only be performed when the female dog is around 5 or 6 months old, before the first appearance of the estrus cycle.
If it is not possible to avoid the chance of a female dog getting pregnant, it is your duty to find a good home for the puppies you are not thinking of keeping. Please don't send the unwanted puppies to a hound.