Every real service dog I have encountered has been trained to be completely unobtrusive.  When I encounter a dog in a business that behaves differently than a service dog (shoving its nose in my butt, jumping up on me, becoming aggressive in the presence of another dog, etc.) I explain the incident to the management and let them know that I will no longer patronize the business until all non-service animals are banned.  Perhaps if more people did the same, we would see fewer people trying to pass their pets as service dogs.