Then come up with ways to create more quality homes! None of this "Oooh! I want to save poor wittle horses from slaughter!" And then they go buy fifteen $50 horses at a kill auction. No. We need more kids and families taking lessons. We need more people looking into and experiencing the joys of horses. We need more owner. More trainers. More lesson riders. More shows. More competitions. More prizes. More INCENTIVE for people to have a horse or five. Come up with ideas to bring back the weekend fun shows. Build up another pony club type thing. Come up with more programs to get people to try riding.
There HAS to be an outlet for the horses, or they will ALL suffer.
Ideally, when there is an outlet for the horses, the number sent to the slaughterhouse will drop.
I definitely agree with LadyDreamer. Making the horses we have more desirable is a great way to cut back on the numbers going to slaughter. (After all, would you slaughter your pal? The horse that you spent years with riding, and loving? Probably not.)
Unfortunately, not everyone has the funds or resources to own a horse, and some of the people that don't still DO.
Recently I heard of a man arrested because his 15 or so horses were living in dismal conditions. They had been out of hay for weeks, and he did nothing to halt the reproduction of his herd, so there were a few foals too. Quite a few had to be euthanized due to his inability to provide for the horses.
And trying to say that this is an isolated incident is, unfortunately, just lying to yourself. There are backyard breeders by the droves, and they all have "thee #1 [unpapered/mutt] stallion/mare" that they breed, and breed, and breed... And then they wonder why no one buys the poor things when, conformationally, they're a wreck and they might not have been handled. People see the dollar signs, and not the true value of the horse itself. Then they get into trouble because they have a dozen horses that no one wants, and that they can't feed. So they dump them somewhere.
This is the unfortunate consequence that is the uneducated breeder/horse owner. Working towards educating horse people on breeding QUALITY animals (as opposed to quantity), and how to care for their animals, would be a better option, in my opinion.
Prevent the problem. Don't try to stop the flow, because the river will always find a way around the barrier.