DROP TABLESPACE tablespacename
DROP TABLESPACE removes a tablespace from the system.
A tablespace can only be dropped by its owner or a superuser. The tablespace must be empty of all database objects before it can be dropped. It is possible that objects in other databases may still reside in the tablespace even if no objects in the current database are using the tablespace.
To remove tablespace mystuff from the system:
DROP TABLESPACE mystuff;
DROP TABLESPACE is a PostgreSQL extension.