Has anyone considered that people that don't post and don't participate may just be here to enjoy the artwork and they might just be favoriting artists whose work they enjoy? Yes, I agree that harvesting has been a problem, but we can't paint everyone that masses favorites with the same brush. Blocking someone from "favoriting" your images won't stop the harvesters. They'll just resort to keeping lists on paper and using the gallery search feature. 

Karen1573's suggestion is the best. Allow it on a per image or per gallery basis where you choose to allow an image or your gallery to be favorited or not 

It's all fun and games...
Until the flying monkeys attack!!!