-What do you mean by trying to injure and is this your first time owning ducks? Many new duck owners are appalled at duck mating behavior and think that it's brutal initially. Ducks do not have hands so they use their bill to grab down on the neck holding the other duck down to ground or even hold their head under water until the deed is done. However, unless they are actually pulling feathers or causing actual bodily harm it is normal mating behavior.
What is your male to female ratio? Typically 1 male needs at least 4-5 females minimum. The exact number does vary according to the drakes personality. If you do not have enough hens for the drakes they will end up over mated in which case I would suggest building a separate drake pen/coop or running a divider. If your ratio is correct and your drakes have not actually injured your hens then allow them to mate.