How much of a military presence do we really need, in order to enforce regulations over shipping traffic though? Surveillance by spotter planes / drones, combined with the threat of legal action against the ship's owners, ought to deter all but the most hardcore bastards.

For them, in theory, a rare single cruise missile from Norad ought to suffice. Even then, you could start with a warning shot, and bill the ship's owner for the missile.

For more complicated missions, we could fly out a small inspection or police team in a flying boat. Instead of relying on arrays of 20cm cannons for protection, they would depend on the above mentioned system (which is similar to the Norad system already in service).