You have to be there to head them off in my experience. I have two directions they cannot go (neighbors) and my yard & a big field where they can go. I am no expert, but had trained my first flock (bought as adults & used to roaming) and now my second flock (obtained at 14 weeks) to NOT go in the wrong direction. I did this by heading them off when I saw them meandering. Mine learned in a short period of time, by me not letting them go there ever! It will be harder if they are into the habit. I would only let them out when you can keep watch on them until they learn. I barely watch mine now & they have been really good. I did use a cheap driveway alarm with the first ones, so I knew when they headed that way when I wasn't watching. It was $14 on Amazon & really helped. Haven't needed it with this group. It can be done, but it takes some patience. Hope this helps.
From my experience, guineas just HATE ducks. Its especially bad in the spring. They chase ours all over the place.
You can sometimes train them when they are young and first start to leave the coop by following them with a walking stick and warding them away from areas they aren't supposed to go. But really, trying to tell a guinea what to do is like shooting rubber bands at the moon.
Good luck, over time, the guineas may just learn that THIS is their space and THAT is the ducks space and let things be. Or maybe not.
I really don't know. I have 15 guineas and 8 ducks.......and in my barnyard, the ducks rule. I've always heard that if you have an adequate amount of guineas in the flock (preferable at least 10 - more is better) they're too busy chasing each other around and leave chickens (and ducks) alone. My suggestion is to get more guineas in that flock of yours.