Sorry this is going to be a really long question! I live in the UK, so I know things may be different here than in the US.
I would suggest checking for rules/laws about wild birds, first. Because you getting legal trouble would not help the ducks at all.
I'm in the USA, and I'm pretty sure that what you suggest would not be permitted here, if the duck really is a wild species.
After checking what's allowed:
If you want to make a big, safe enclosure and shut them up in it: watch how the mother reacts, and make a decision based on that. If she walks around quacking, then settles back down, that's fine. But if she constantly paces and fusses, there comes a point when her stress is bad for her and the babies. And of course, if she steps on them in her distress that could kill them too--which isn't really better than having the other wildlife kill them.
Have you considered just taking her eggs away? (Subject to what's legal, of course.) She's already had one brood this year, and no ducklings means no dead ducklings. You could leave her an empty nest and see if she gives up, or you could give her fake eggs and let her sit on them for a while.