Databases are destroyed with the command DROP DATABASE:
DROP DATABASE name
Only the owner of the database (i.e., the user that created it), or a superuser, can drop a database. Dropping a database removes all objects that were contained within the database. The destruction of a database cannot be undone.
You cannot execute the DROP DATABASE command while connected to the victim database. You can, however, be connected to any other database, including the template1 database, which would be the only option for dropping the last user database of a given cluster.
For convenience, there is also a shell program to drop databases:
dropdb dbname
(Unlike createdb, it is not the default action to drop the database with the current user name.)