How to stop our runner duck from pecking hens?

We've raised 5 hens and 5 runner ducks together now for almost a year. Recently, our only male duck has begun to harass a few of the hens and show mating behaviour towards them. One hen now has exposed skin on her neck/back where the duck has been nipping at her. All have very ample room to forage during the day. Besides culling the male duck, what can be done to stop this activity or provide additional protection for the hens? A commenter on another recent thread recommended a 'pick no more lotion', available at: http://www.flemingoutdoors.com/rooster-booster-pick-no-more-lotion.html.