I like your idea about signs in Spanish, warning about the deadly effects of falls from that height - both from a humanitarian and a deterrence perspective.
How about “Cuidado! Las caídas desde esta altura son fatales para la mayoría.” (Beware! Falls from this height are fatal to most.)? Or maybe “Estas Caídas Matan!” (These Falls Kill!)
Then an artful depiction of a stick figure burning in through the atmosphere for a fearsome impact, or horribly twisted on the ground.
Word will get around after the first few of these tragedies, and they will be covered in Mexican media.
Of course rumors of the random subsonic strafing runs would probably travel pretty fast too.
(what if they were to break the sound barrier that low?)