It's the kind of problem that has been plaguing online artist communities for decades now, which is why most sites give an actual copyright to their members' submissions. I'm pretty sure Creative Commons gives the original creators of the file a copyright on the design, but actually filing a suit is costly.
Then again, defending against a suit is costly too, and threatening to file a copyright lawsuit is basically free. If it is just some kid in their basement, a flood of properly written C&D letters ought to scare them away posthaste.