What has worked for us is a shock collar and a firm NO. As the dog would approach the flock I would say no and watch. If they kept up the approach they'd get a shock at the same time I said no the second time. If we had the same issue again in that training session the shock was on the first no from that point until we were done training.
We have a doggie door so sometimes I peek out the window and watch and reinforce with the littlest one but there has been much improvement. Especially now that she knows I can reach out and touch her even when I'm not right there in the yard.
