If they are using your files, you have a copyright on them which automatically comes into existence when you created the files and published them. They are in the U.S., so you can sue them in the U.S.
Under U.S. law, it is important to register your copyright with the Copyright Office (Library of Congress). If you do this in a timely manner, which means less than three months after publication, you can sue in federal court for statutory damages of up to $150,000 per file for willfully redistributing the files. Suing in regular federal court is difficult and expensive and takes a long time and you would likely need an attorney to navigate the process. There is an alternative, however:
You can also sue via the Copyright Claims Board at ccb.gov which has a simplified speedy process and which can get you up to $15,000 per file (with a limit of $30,000 per suit). Note, I do not know whether you need to have timely registered your copyright with the CO/LOC to do this. (You DO need to register it, though, even if untimely. I don't know how an untimely registration affects the damages you can get with the CCB.)
Registration with the CO/LOC is simple but does require paying a fee. Last time I went through that it was $40 per registration.
You can talk to or even hire an attorney to help you navigate the CCB if you really want to, but AFAIK it is straightforward enough that most people can navigate the process on their own.
Edit: I noticed below you mentioned you published under a CC license. You still retain copyright, CC licenses just tell people how they are allowed to use the files, and is explicitly not public-domain. However, that may or may not complicate things with the CCB and/or regular federal court. You might want to talk to an attorney familiar with copyright law and Creative Commons licenses to figure out how much you can sue for.
Another edit: note, I haven't tried registering STLs, STEPs, or other CAD type files ever. I assume they can be protected the same as software programs, but I don't know this for certain. My work was mostly with commercial packaging (to be able to block counterfeiters).
Disclaimer: these off-the-cuff remarks are not legal advice. For legal advice upon which you can rely, please contact an attorney licensed in your jurisdiction. These remarks were made using 100% recycled electrons.
"superior" depends on the application. IMHO CF should never be used for something that will be handled regularly with bare hands. Also, IIRC the GF stays unsoftened to higher temperatures than CF, which helps when being used for e.g. a barrel mount. I remember a video of someone doing a mag dump on a presumably PLA frame where the barrel was flopping around like a freshly caught fish's tail after a few dozen rounds.