It will be difficult to nearly impossible to keep the hawk away not that it has had a successful kill. I think that the best solution would be monitored/guarded free ranging and maintenance in a pen at all other times. Good luck at reaching a resolution.
Welcome to BYC! You might want to start a new thread in the Predators/Pests forum: https://www.backyardchickens.com/f/13/predators-and-pests but unless they are kept in a covered run like sourland said, there is really no way to keep them out of your yard. Good luck to you.