Can the Oracle database schema be cleared without dropping?

In short, you cannot. There is no ALTER USER DROP SCHEMA command. You can drop the user and thus drop the schema objects. If you want to drop all objects in the schema, you can do a simple operation as follows:

SPOOL drop.sql
SELECT 'DROP '||object_type||' '||object_name||';'
FROM dba_objects WHERE owner='SCOTT';
SPOOL OFF
@drop
@drop

The above quickly creates a script to drop all of the user's objects. It then runs that script. The script is run twice to handle objects with Foreign Key constraints. But the above is not a single-shot as requested.