Don’t offer them ATC services?
“USAF6969 Land at the nearest suitable airport immediately, you did not have clearance to take off.”
Don’t comply? Then scramble fighters and threaten shoot them down if they don’t let themselves be escorted to an airport. Like is protocol for every country’s airspace.
They do that for Russian planes, even civilian ones.















Yes? Countries can ground planes. That’s how airspace sovereignty works. This was a choice on Spain’s part.