[Help] My dog attacks other small dogs, how do I stop it?

I have a japanese spit. She is very friendly and lets strangers pet her, she likes little kids, and she likes to play with other dogs just fine. But as soon as she sees a god smaller than her she goes into hunting mode, she crouches and hides waiting for the dog to come close, and then she attacks him, she doesn't even bark, she straight up wants to kill them.This happens everywhere, far or near the house, no matter who is with her. The only way to stop her is to get in front of her looking her at the eyes or putting her on a leash, but now every time we go out I have to be on the look for small dogs.The worst part is that I don't feel like she's trying to protect us nor anything, she seems to do it as a sport, because is fun.I have tried to nag her or punish her when she does this, it worked for training her for everything else, but not this one thing.I wait your advices via /r/dogs http://ift.tt/2s0REhY