it's sad but there is not *much* we can do. To me brining awareness to everyone possible is the best we can do. People don't but from a petshop because they are evil and are willing to support puppymills, they get them because they are cute puppies who need homes and the buyer has no idea of the horrible situation they come from and that they would be supporting.
I think if every one was educated the number of people who buy from them would drop drastically. I also think the only ways we could teach it to every one would be schools and TV, preferably the news and children's tv (some how that is not too gruesome).
If the ASPCA finds a place that they think is a puppy mill then they do shut it down. There's so many and they are usually no where near other peoples houses so people can't report them.
Byb's are harder because *usually* the animal is not suffering and is living a pampered life. The thing is they are still irresponsible, you are not getting your money's worth and the dog is not the healthiest he can be. So people think, well the dogs are living inside and are treated well, they even go to the vets!
so what is wrong with them? People need to be taught why they are bad and what a good breeder is.
If no one buys the designer dogs from the breeders, then the fad will die sooner. That will get rid of half the BYB's who do exclusively designer dogs. When the BYB's are not getting the income from the puppied they might stop. For them though there needs to be regulations in place, from the law. The most important things when I look at a breeder is do they show and are their dogs champions? and have the had health screenings (OFA), and are the dogs healthy. maybe the breeders should be given a certificate for each dog when they win a champion ship stating that this dog can be bred and if you are caught breeding without the certificate the dogs get removed and you are no longer allowed to own intact pets.
I don't really know how you would go about doing this though, it's just a theory.