Postgresql: No data left on drive, how can I wipe it and start over?
Since you say that the config directories are in /etc/postgres I'm guessing you're running Ubuntu or Debian, which use pg_wrapper for management of their PostgreSQL installs.

On these platforms you use pg_dropcluster to destroy and remove all databases, then pg_createcluster to create a new blank server instance.

This guide for PostgreSQL on Ubuntu may be informative.