they have a bad history with pets say something like, im sorry but with your pet history i would feel better giving my pups a home with a diffrent family.
they have kids and you dont sell to people with kids say, im sorry but we dont sell to familys with children, im sure your kides are great but puppys can easly be hurt when you kids are (pick a age) if you still want one let me know.
they dont have a fence it can be, no matter how carefull we are dogs can get away from us, for this reason i only sell to people with a fenced yard for the dog to potty on. it reduces the rick of them running away being hit and being stolen.
now if you just dont like the people and dont want them to ever have a puppy from you you can say something like. im not sure your family and/or lifestyle is right for my puppys i hope you understand but i can not sell you one.
if its the first time they are being turned down for a puppy its going to bug them no matter how you put it, but they will get use to it.
just rember they are your pups, its your job to make sure they are in good homes, and even if you do upset people thats ok because your pups will be better off because of it.
I'm gonna trade this life for fortune and fame I'd even cut my hair and change my name