Even in the old days, a few people would try to get copy prints made from the proofs at their local photolab. That is one of the reasons that some pro labs imprint copyright warnings on the back of every proof.
There really is nothing you can do, practically, to eliminate this. If people are prepared to accept a lousy copy in order to save a few bucks, they'll do it.
That being said, it sometimes helps to include a "free" small print with a more expensive enlargement.