Have a link on the Dashboard page for "Delete Account", which will bring user to a page where they enter their password and check a box to verify that they want to delete their account. Deleting the account should delete all of the users posts and remove their membership.
If that was a feature that came out, I'd want control of being able to not have it available. I don't want users being able to delete their own posts! Every post helps my SEO.
