The is a fairly standard scenario in my games. I destroy a pirate group, and as soon as this is done any nearby empires already have colony fleets en route (computer player omniescence ). I leave the attack fleet in orbit over the worlds I have cleared, awaiting my colony ship. Their fleet arrives, I intercept and capture / destroy it. There should be both a diplomatic penalty for destroying the fleet, AND an ability to warn them off. IE we claim this system, attempt to land and steps will be taken.