The biggest drug problem in the US today are opioids and the only thing that will "cut down" on that problem is to start closing small town drug stores where millions of pills are being dispensed.
http://www.nbcphiladelphia.com/news/local/Delaware-Sues-Pharmaceutical-Companies-Drug-Store-Chains-Over-Opioid-Crisis-470171953.html
And then there's the addition by express mail:
http://abc7chicago.com/opioids-overnight-addiction-by-express-mail-/2989587/
How is a 'wall' going to stop these sources of drugs?