Databases are destroyed with the command DROP DATABASE:
DROP DATABASE name;
Only the owner of the database, 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. template1 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: