Teach her to one rein stop if she doesn't know how to do it yet. Vida had no "whoa" when I first got her and that was the only way I could get her to stop.
In the mean time, work her with a rail or fence line. Walk her up to the barrier, sit back in your saddle and say "whoa" or halt as she nears the barrier. Do it over and over till she figures it out.
There are tons of ways to teach this, this is how I did it. It worked great.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France