Oh I had similar experience happen not too long ago.
I had a woman come over and to look at pups. First she knows nothing about the breed. She asked why the pups were all black and tan. I tried to explain to her that all yorkies are born this color and then as adults their coats turn blue or silver as some call it. She points at one of my blue and gold girls and says "but I want one like that". She clearly was not grasping the concept. She then tells me that she use to have teacup poodles a few years back and that she bred them as they were very desirable and now she wants a teacup yorkie.
I told her sorry but I would never sell her one of my pups.
The nerve of these people, where do they come from?!
I too have a spay/neuter contract. Mine states that they must have them spayed/neutered by 9 months old and I have full rights to confiscate the puppy if it is not upheld. Sometimes it is a little later than 9 months as I've kept a puppy longer but there is still a good time frame and it was only with boys. If I kept a girl that long she would be spayed before leaving.
So far I've had no problems and everyone has followed the rule but it is something I worry about a lot.
Oh and I do have in my contract that they aren't allowed to rehome, give away, sell or abandon the puppy/dog, they have to come back to me. And they can't euthanize without my permission ever.