It could take months or years for an asteroid strike to render a planet fully uninhabitable, and in a universe with large spaceships and hyperdrives, that means a reasonable survival rate via evacuation. So now you've committed to months and months of blockading the planet to prevent evacuation. At that point, why not just blockade the planet in the first place and wait until they run out of some sort of vital food/medicine/defense goods or the population just capitulates out of war weariness?