You explain to them nicely that they will be arrested and/or put on a no-fly list once they arrive at the next airport. It's the same with anyone breaking other rules on an airplane mid-flight - such as smoking, vaping, spritzing perfume or various other activities that are normal in other situations but illegal on an airplane.

How do you think the mask mandates were enforced mid-flight when they were universal?

Also, everything in saying is already common practice in much of the world, especially in East Asia.