If the caravan army continues on foot, they soon run out of food and supplies. They are forced to turn around and go back.
Northern supplies are cut off. They will march where the food is located.
Bomb a few key roads per day. The longer Mexico waits, the fewer Northern passable roads.
It is simple, and the best solution. Nobody gets hurt.
But bomb early.
And Mexico will gladly allow the US to bomb roadways in their sovereign territory?
On the other hand, one could tell Mexico that there will be a hefty tax on remittances to Mexico - a tax that increases the less that Mexico does to stop the invasion.