So I was thinking about the way he worded this, 25% tariff until the border crossings stop. I think he might be onto a 4D chess move here.
If he tanks the US economy so badly that it's financially detrimental to cross the border then the flow of people across the border will definitely come down. If he tanks the economy hard enough we may even see the flow reverse, at which point he can lift the tariffs and declare the job done.
He is already happy to make Americans ignorant, illiterate, and spiteful to outsiders; is he really going to baulk at making them dirt poor as well?