Got to say I agree with Riverman. I post a lot of photographs on FB, not so many on here now, and I accept that there is a good chance that someone will steal them at some point. I post for my enjoyment, and that of others, and accept the risks that go with it. I don't think you'll ever stop it now, the genie is out of the bottle, in some ways it's flattering if someone thinks your photographs are worth stealing. It's got to be worth considering if trying to push back against this anymore is worthwhile. If I have a photograph that I really want to keep the copyright of, I wouldn't post it on the internet.