I personally have had problems with this the whole two years I have been on the internet. My art being stolen constantly, if anything, the only thing I let others use is line art, which would mention FREE USE obviously. But I am seriously touchy when people even think of using my art at all, specially because all my art is my characters or a request for a friend. I have my own website, people steal from it all the time and think it's even clever to steal my name, personal information and every thing. Content theft in general drives me up the wall and I see no excuses for it.
I haven't had trouble asking them to remove it right away, only twice have I actually had to report some one for throwing a fit and refusing, one even tried as hard to say I was the fake. e.e I have had people even steal the exact details to the character as well, which in my opinion makes it worse.
I love making art for others, when I get the chance, and that's the only time they can use my art, while still giving me credit.
And also I'm pretty sure it is correct, you do not need a warning or water mark on your art for it to be copyright, it is technically yours either way, the owners property.