"Leave it" is my first warning. It works for me 95% of the time. Say if someone is walking down the road and Hondo locks onto them. I'll say 'Leave it' and he'll turn and look at me. He'll keep glancing at them as they walk by, but he won't lock into them. While they are walking by I might add a few "good, leave it, good!" if I think he is beginning to engage (like if they have a dog that isn't on a leash running around with them).
'Leave it' is most likely the best command I have taught him.
Hondo Von Dopplet L Bauernhof "Hondo"- GSD
Lilie's Tug McGraw "Tug" - Golden Retriever
Maggie - Mini Dachshund (Rescue)
Lonestar Pivo - Texas Blue Lacy
Ashe - Barn Cat
Katie / APHA