When you find an enemy planet, double left click on it and select details. Then select demolish all improvements, or better still, destroy colony. Its gone.....

Select an empty planet, select details, game locks up solid.
Double left click on an enemy ship, you can now set the ships action code.

Buy a planetary improvement, save the game and reload the save. You can now buy another improvement. I did this for 20 factories, all in the same turn, same planet. GalCiv2 0.77b