I totally understand. Had a lot of issues with people using pix for Ebay. I learned to get Ebay's attention, and they deal with is pretty fast. The galling thing is that most people doing it will send a nice letter/e-mail of apology and 10 minutes later, do it again. Or claim the pix were from a friend, etc. And usually it is a nice pix of something different than what they are selling, with is fraud and deception, plain and simple. When you contact people about the use of your material, "fraud" is always a good term to include.
I have no issue with people using the material for educational purposes, but not commercial purposes.