I was selling a mare once, and a girl came out to see her, rode her around, and her dad said that they wanted her. Well seeing the girl riding my high strung 11 year old Thoroughbred, I could tell that there was no way it would be safe for this girl to have the horse, nor did I want to sell the horse to someone who might do the same thing to her that her previous owner had, letting her sit for a year because the daughter couldn't ride, then decided to sell her but needed her gone "now", so told us she was going to auction if we didn't buy her by the end of the week. Of course she turned out to be an awesome horse for me, but beside the point, so me and my dad talked, and my dad politely explained to the man that he wasn't sure that my horse would be the best fit for his daughter, being that my mare was sometimes unpredictable, and needed a more experienced rider, to make sure both horse and rider were safe. The man was thankful for our honesty, and they went elsewhere to find a horse, with no problems. I wouldn't want to jump into a "you aren't good enough for my horse" type of argument, but if you truly think that this won't be the best home for the horse, don't be afraid to just say that you don't think it would be a good fit, and if possible, point out another horse that might be a better fit for that person. Not that we want any more random horses being bred, but at least that way it puts her focus in another direction while you are telling her she basically can't have your horse.