To end your membership at the Nation, you contact the First curator with your errand as well as name and personal number. Another option is to not pay for the membership in which case it will end automatically after a while. You can also end your membership by visiting the curators office during the office hours.
If you are a member of more than one Nation, but only want to leave one of them, you have to contact the specific Nation to end the membership. If you don't, the unpaid fee will stop the Nation card from being activated. No Nation can end a membership other than their own.