Not meaning to burst anyone's bubble, but you actually can if you know how to do it (and it's very easy to do once you know how, btw).
With that being said, the only real protection you have online - even if it's on your own website - is documentation. Make sure you keep all your original files, past those of the upload date - because the file creation date can make or break a fight of who owns what. Along with that, having the .PSD files (or whatever file you have) with all the layer information, hidden watermarks, etc. is also a weapon you can use to its full advantage.
Either way, don't give the baby its bottle in this type of situation. Know full well that you have all the cards in your hand right now - from the file creation dates to the upload dates to the fanbase that can speak on your behalf to the raw files themselves - so if she tries to pick any more of a fight with you that threatens your online persona/reputation, then she'll have a pleasant surprise waiting for her. For the most part though, just leave her be - attention is what fuels these sorts of people, and if you give them all the attention you have to try and stop them, it's just negative reinforcement (which is what most kids that do this are looking for). Let them realize on their own that nobody is going to give them attention; they'll move on to something else, and hopefully, eventually, they'll get the wake-up call they need that what they're doing is pointless and shameful (whether it's a lesson taught by maturing or by another creator/platform/authority giving them the punch in the teeth they need).
If it really starts to damage your rep, as others have said, you have a fanbase that knows who you are and knows that you are the only one that provides the content you do.
However, do not use your fanbase to bully anyone. I've seen people who do this and shit always hits the fan whenever you do (and the creator just ends up looking pathetic). Once you use your fanbase as some kind of personal army, you've lowered yourself to the same level as the person harassing you. Your fans are proof that you are the original, but they are not pawns to carry out your every beck and call. They are there to confirm that you are who you are if anyone asks, and the strength is in the numbers and time span, but the moment you use them to harass or fight anyone else, you are no better than the ones that actively harass those to get what they want, whether it's stealing art or bullying creators, whatever it is. It's not so much a warning as it is a precaution for your own sake, and something like that can hurt your reputation more in the long run than any single kid copying and pasting your art could ever do.
