It's really unenforceable on a large scale. I would imagine there will probably be legislation to require production 3D printers to have some kind of failsafe baked in like anti-counterfeiting measures in scanners/printers so the average joe who buys a printer off of Amazon won't be able to do -- they'll have to build a 3D printer running on open source firmware, but even then parts will likely be regulated like chemicals are regulated for drugs.

They'll make an example out of people like they did with the war on drugs. It won't stop people from doing this, but it will definitely push the community further underground and criminalize people who shouldn't be criminalized. You'll hear about dudes on the news periodically who got caught "Printing ghost guns for domestic terrorists" and they'll treat them like they got caught with CP.