Bendix cannot sell aviation radio systems unless they are tested and ticketed by the aviation authority of the country in question. So we legislate the same way for drones. A manufacturer must have X Y Z features (with a good mind to making any measures as unhackable as possible, or more realistically, unhackable to 90% of the population) or it is not allowed to be imported or sold in the countries market, period. Buyers do not assume the risk of being caught with unregistered kit, the legislation can be broadcast in such a way that it educates flyers about airspace and separation safety, and if operators are indeed found to be in breach by hacking around measures designed to keep the hobby safe, then 10 tonnes of solid, legislated rape can be dropped on their heads. This also has the effect of encouraging manufacturers to build the systems in and build them well, lest they find they suddenly loose access to a market.
Yes, there will be grey/back imports but again, this is about setting up a workable framework that will be effective 95% of the time.