kinda like asking what do I think should be done about illegally buying a real gun, no? I don't think it's possible to totally prevent people with poor judgement or malicious intentions from getting their hands on a weapon, but if you prevent most people that shouldn't own a weapon from doing so, you're already on the right track imo.
also, I'm not saying I know the exact solution for every aspect of the problem of gun violence in the US, like 3d printed guns, or even if a certain solution will work but how many mass shootings involved 3d printed guns? it seems like it could become a serious problem, but there are more burning issues right now like how easy it is to just legally buy a weapon in the US.
maybe make it illegal to 3D print a gun and build a system that recognizes and prevents the 3d printer to print it, but even if that's not possible, or there is a much better solution that I can't think of right now or at all, I think that the more difficult you make it for people to get their hands on weapons, especially semi auto or automatic weapons, rifles, etc, it's still better than the shitshow they are in now.