“There is at least one more bear trap on my grounds than you can spot.”

"Entry Fee (min one):

• Carrot
• Apple
• …"

That way you get duck (or human) food for free.

depending on where you’re from you can hang up a sign saying “trespassing Children will be converted to Islam”

Trespassers will be trebucheted.

Get you some geese

Pro tip - feed them a handful of dandelion leaves twice a day, they’ll be your best friends

You’re looking at this wrong way.
Buy a gumball machine. Put on fence. Fill machine with enough feed for the day. Charge other people to feed your ducks

Build an exact replica of that area right next to the original. They’ll be confused, are they visiting the real ducks or a simulacrum of the ducks?

Obvious answer is to give the kids ducklings for every visit. Pressuring kids parents to stop their kids coming over without permission. Unless they want to own duck pets.

Give the ducks a nice shower/sprinkler set up that’s both timer and manually controlled.

train the ducks to chase after the enemies. Lol

I would have to go with landmines. If that isn’t enough of a deterrent, and claymores or even some bouncing betties.

Put up a sign that says “WARNING: rabid ducks, enter property at own risk”