Disclaimer: I'm not a pro. Photography is a hobby for me.
I have had a number of photographs purloined from google searches and Flickr etc. I had my easily removed watermark in the bottom right corner cropped out. The first couple of times I was flattered. Then after a few times I reacted to it and had a range of responses that went from apologetic all the way up to total douchebag. I started sending invoices for what I arbitrarily deemed market value for the image(s). A couple actually paid. Most of them removed the image. Whatever the result I accepted it. The send an invoice tactic was suggested by a lawyer friend.
The moral of the story?
As long as we post up awesome photographs and make it easy to just take high resolution/quality pictures with a right click and a 4 second MS Paint edit, people will just take them. Just like they do with movies, music....anything they can find online that is available freely in a digital format. Don't take it personally. It's a cultural thing. The internet makes it easy for people to be douches.
Maybe if 'everyone' put larger centered partially transparent watermarks on the images they posted online....it would curtail this free-for-all. Maybe. I stopped posting everything to Flickr. I added image harvesting countermeasures on my site (not foolproof). I stopped posting all the pics I take, showing only a smaller sample set. As a result, I have had much more requests to purchase images/sets of images.