If you place pop in spaceport without destination, that pop will remain there until you move again to a planet or select a destination.

If the destination cannot be reached once they're on the ship traveling (i.e. you lost the system), then they'll be killed.

Think this is what you're asking.

No i get that part. what i want to know is this for example. say i have a system with 3 craver pop. if i move them to soace port with no destination, do they still have a chance of being born in my system?  if not then soace ports can kind if act as holding cells for pop you dont want growing on your system. 

Tested it out innonenof my livestreams. Placing a population pip on a transport shuttle with no destination does indeed prevent it from being choosen for birth on a system. This is definitely a potential waynto help groom systems for the pops you want and prevent the ones you dont from growing, as long as you can completely remove their numbers into shuttles.