You kill them before they get here by breeding populations of sterile flies and releasing them into the wild to the point it disrupts their reproduction cycle. I'm not sure how many reserach/breeding facilities we even have left, maybe one in Florida still??, but I'm sure they're ramping up production and dumping them all over southern Mexico.

The Darien gap was left as is for a very important reason, and it wasn't just to control the flow of the two legged critters. It's a shame they've been allowed to move north of it, but throwing some money and bugs at it over time should put it back into the column of something we don't have to worry about.