Hahaha holy crap. These messages are so desperate.
You can't stop people from being horrible on the internet, but there's nothing this person can REALLY do -- that's why they're so desperate to extort you and convince you they know how to hack your account ( haha what ), because reposting your art isn't sustainable for them. They are a headache trying desperately to be more scary than a headache. They want to believe they have a gun to your head, but it's just a kidsafe nerf gun.
You basically can either ignore this small child, or tell her calmly to stop messaging you. I don't know that you want to get in the mud with her and threaten legal action unless you know how to actually do so -- if you're bluffing, that might escalate this foolishness. I don't think there's anything wrong with telling your followers "so-and-so is impersonating me," if you think it's really necessary. But there's something to be said for giving her no attention for her trouble if it's not. Sending an artist 15 threatening messages and not even getting a mention on their page for your trouble has gotta be discouraging.
Basically, I think it depends on how much of an actual problem for you this person is in their thievery. If it's just obnoxious, then just report them. If it's actually a problem for your rep, then go ahead and let your followers know what's going on while you report them. That's all I would do.