DROP USER name
The name of an existing user.
The message returned if the user is successfully deleted.
This message occurs if the username is not found.
You must drop the database first or change its ownership.
DROP USER removes the specified user from the database. It does not remove tables, views, or other objects owned by the user. If the user owns any database you get an error.
Use CREATE USER to add new users, and ALTER USER to change a user's properties. Postgres comes with a script dropuser which has the same functionality as this command (in fact, it calls this command) but can be run from the command shell.
To drop a user account:
DROP USER jonathan;
There is no DROP USER in SQL92.