To manage the groups you belong to, make sure you're logged into your account and click on the account dropdown at the top of any page. Click 'Settings,' then on the next page select Email Updates.
Under each group's name, you'll see a Leave this Group link. Click this if you no longer wish to be in the group at all.
To the right of each group's name, you can click the Email and notification settings link to manage the emails you receive from that specific group. Make sure to click the Save settings button after making changes to each group.
If you need to leave a Meetup Group, you can always find a new one! Just go to the Find a Meetup page to search by keyword for anything that interests you, wherever you are.
If you would like to remove or delete your account completely, and no longer be a part of Meetup, you can do so by hovering over Account and clicking General.
Scroll down to the bottom of the page and click the Delete your Meetup account link. Be careful, this is permanent!