When it comes to private clients, the best way to prevent copying, is to charge (and make your money) on the shooting, not on the prints. I know a lot of photographers do the opposite to keep the initial cost for the customer low, but a lot of people likes the idea of paying more for the initial shooting, and then not having to worry about expensive prints.
Dealing with private clients is something I just do every now and then, but when I do I simply give them a CD with high res images. I then recommend the lab I use for making the prints.
Chances are of course always that someone will use that CD to make some horrible kiosk prints, but I think the chance of that is smaller when they have a high res original, and can have nice prints done for a rather low fee. It will at least keep customers from scanning poor sample images.
If the client wants me to make the prints, I simply order them from the lab, and adds on a percentage on top, but they are still very reasonable priced.
As for the legal aspect: It is not illegal for a private individual to copy a photo for their own personal use. It is however illegal for a professional or 3rd party to do this or for a private person to do it for profit. So if you use your own scanner and printer at home to do it, it is legal, but if you bring it to a copy shop to have it done, it is illegal for them to do it.