Suggestion C - Privateers : Privateers can attack your civilian ships. So, you can keep some pops in spaceport without killing them.
Suggestion B - Indefinite Waiting : The population in spaceport without any valid destination will indefinitely wait in spaceport. Send all the unwanted pops to the prison system and constantly use the chain gang there. + If you have a system with 1-2 planets, use it as a prison system. So, when an unwanted pop is displayed as a next pop growth, use chain gang. The way chain gang works is quite similar with population growth. Suggestion A - Chain gang : You can chain gang the population if your manpower capacity is not reached.
