I think LLY & NN's easiest target is the thousands of medspas selling their product. Next would be pressuring the CN government to shut down the gray producers. Packages will always be able to get through customs, they just don't have the manpower to check every package. Seizure rates will likely go up, but peps will continue to flow if there is a company in CN producing them.
People often make the claim that peps are similar to the CN governments efforts (and failure) to shut down the flow of Fent and precursors into the US and Mexico. Without getting too political, there is zero incentive for CN to shut down the flow of a drug (fent) that is toxic to the US, causes unrest and is a genuine drag on our society, From the economic cost of policing, medical system, jails the breakdown of families etc. The US and China are frenemies bordering on enemies and the disruption caused by fent is widespread and reaches every corner of our society.
GLP's on the other hand are good for our society, they make people healthier and reduce the burden on our medical system from the dozens of diseases that are exacerbated by obesity. The CN government has incentive to curtail the gray industry before LLY and NN even start pushing for tighter controls.