Nothing much you can do, as mentioned above, unless you make the photos small, low quality, and put a watermark in the middle of the photo, which doesn't help to display your work. However, I do watermark photos I post to social media sites such as Facebook, because of their terms of use, which say they claim the right to distribute the photos to third parties without compensation to, or notification of, the photographer. It is illegal to remove a copyright notice in the US at least. This will not necessarily stop thievery though.