Well, for one thing they could impound the vehicles of the drivers caught transporting the illegal immigrants.
The average driving distance from Guatemala to the US is 2100 miles. The average transit time is 25 days. Gentlefolk, they are not walking here.
Mexico claims that they don't have funds to stop the tide.
Impounding the vehicles and selling them at auction generates revenue. It also sends a very clear message to Jose Average that helping an illegal can be very expensive.
Just doing a little bit of thinking outside the box.