This is not a digital problem. People paid to have copy negs made all the time for the same purposes before scanners were commonplace household items. I certainly had my share of them made to protect family photos that existed as prints only. I agree with Steve and Matt. There is not much you can do if they are willing to settle for the quality they get by copying proof prints. The best way to fight it is to choose your clients wisely. Working for scumbags is rarely worth it, no matter how much money they are paying you.