Now that he's come once, he'll come back till they are all gone. Basically, your ducks are "The Old Country Buffet" for him. Go to Premier 1 Fencing. They have electric netting that will at least keep them safe from everything but Hawks/Owls. I would strongly advise to get higher than the 42" ft poultry netting. We got the deer netting with 3 inch space between them that is 68" tall. See:  
https://www.premier1supplies.com/deer/fencing.php?fence_id=115 You can get solar or electric charger. It MUST BE ELECTRIFIED to work! We got enough to fence the garden for when we want the ducks there and it's a HUGE area, 165' X 55'. They are totally happy. But even then with the Hawks, we sit out there. We're waiting on a Pyrenean Mastiff, giant sized guard dog. The other thing we use when we aren't going to sit with them is a movable Chicken run from Stromberg's. See: 
https://www.strombergschickens.com/product/walk-in-chicken-run-13-1-ft-x-9-8-ft-x-6-6-ft It's really not safe for Foxes, but we have a dozen dogs that keep foxes away in daylight hours. Ducks are happy little dudes, and they don't sit around and think that they have no life because they aren't loose. Ours are so happy with being moved daily.