This is because of trade. Your citizens will migrate without your approval and some idiot thought it would be a good idea to bring a 'friend' home to Earth. Welcome to 'earth hive'!
Seriously its one reason why I never allow open borders with the Festron. I find ships with them? Out into the cold dark they go! I am not even sure I want to conquer their worlds.
Ill wait until we have the means to glass the planet (which is coming). Just conquering them can make the problem worse!