Any spaceship over several hundred tons is an extremely dangerous weapon in the wrong hands, particularly if it's intended for reentry. Made mostly of metal and possibly shaped to be able to breach an atmosphere without trapping enough air and generating enough friction to destroy itself, it'd be at least dangerous as the worst asteroids that have hit the Earth were it sent plummeting into the surface at full burn. Not only that, but it may well be able to hold its own against an armed assault - it doesn't have to be functional or even in one piece to not destroy a town. Even if it is vaporized or broken up, coming straight down in a planned route, the fireball or resulting explosion would as devastating (if not more) than an atomic bomb.
Clearly, any commercial space-faring civilization has the capacity for one lunatic to try and wipe out as many people as they can with their own death. How would a planet protect itself against such a malicious threat?