I don’t know what ‘they’ refers to here. (But whatever it is, I suspect your claim about ‘noticeably hindering its rate’ relies on data existing only in your imagination.)
You’re assuming the wall will make drugs scarcer (and therefore more expensive/difficult to acquire). This is a fact not in evidence. But even if the wall does impact supply in this manner, you’re assuming the effect will be such as to offset the enormous costs incurred in building and maintaining the wall; ie, that the wall will be a cost-effective means of reducing drug dependence. This, too, is a fact not in evidence.