How to remove the APEX option from the database

cd $ORACLE_HOME/apex
sqlplus / as sysdba
@apxremov.sql
drop public synonym htmldb_system;
drop PACKAGE HTMLDB_SYSTEM;

If you have older APEX installations, they may be left in the database, but not registred in the dba_registry.

I had an old APEX schema called APEX_030200, with several invalid objects:

OWNER	OBJECT_TYPE	COUNT(*)
PUBLIC	SYNONYM	3
APEX_030200	PACKAGE	2
APEX_030200	PACKAGE BODY	114
APEX_030200	PROCEDURE	3

This means that the script above won't work. You will get this output when attempting to remove the installation:

sqlplus / as sysdba @apxremov.sql
...
Error:
You can only use this script to remove Application Express

I found some useful information about these situations at this blog

Basically, you can simply drop the old schema directly.

Check first:

SELECT username, 'drop user ' || username || ' cascade;' AS remove_statement
  FROM dba_users
 WHERE     (username LIKE 'FLOWS_%' OR username LIKE 'APEX_%')
       AND username NOT IN ('FLOWS_FILES',
                            'APEX_PUBLIC_USER',
                            'APEX_LISTENER',
                            'APEX_REST_PUBLIC_USER',
                            'APEX_INSTANCE_ADMIN_USER')
       AND username NOT IN (SELECT schema s
                              FROM dba_registry
                             WHERE comp_id = 'APEX');

So cleaning up can be done as easily as this:

drop user APEX_030200 cascade;