I use vocal cues mainly for lunging, but I did have one instance where voice saved a young rider from bouncing off my horse. She accidentally found his "lope button" and was getting a bit sideways in the saddle. I yelled "Ace, whoa!" from across the arena and he came to a nice soft stop. She then giggled and asked how to get him to do it again!! I love young riders, they are fearless!!
For my horses, whoa means stop, trot and walk are just that, and to slow down but stay in the current gait, I use easy. You can use any words you want, just be consistent and make sure the words you use are easily differentiated from one another. Horses are smart, but try to make it easy for them.