The only way to avoid this happening is to catch a few of the perpetrators and slap swingeing fines on them.
I think it was Getty (or maybe it was Corbis) did this a year or so ago. They used a program to trawl the net and identify images that were on their books but being used illegally. Then they send hefty demands to the people using these images.
I heard of several small businesses in UK that were astonished to receive bills of several thousand pounds, completely unexpectedly.
In fact, some of the businesses were not directly involved in the wrongdoing as it was their advertising agency who had stolen the images. The businesses thought it was all legitimate.
But, whatever the rights and wrongs, I believe that put an immediate stop to people stealing images from Getty (or Corbis).
Maybe that's why the thieves are turning to microstock now

IS and SS and all the others should clamp down hard on this, too. Unfortunately some of these examples of theft are on Yahoo and, it seems, Yahoo is so big they don't give a tinker's cuss about copyright violations.