Why would I drop a colony ?
Many reasons can lead you to delete a colony. An abandonned colony can be re-colonized directly giving you another shot at finding characteristics matching your objectives.
You just need to click on your planet's name on the overview :
Then, click on “Delete planet”, the password “GoodBye” is asked to confirm the surppression of the planet, to avoid any mistakes.
The same method is required for a moon suppression.
ATTENTION : you cannot give up your mother planet (MP), neither can you delete a planet with some activity going on (building, fleet, defense in progress or fleet flying from/to the planet).