Even if you delete your application and try to re-import using the same application ID you'll get the message.
To view "reserved" application ids you can run the following query (please note you may need to be a dba user):
select *
from flows_030100.wwv_flows_reserved;
In order to fix this problem you just need to delete the application id from that table:
delete
from flows_030100.wwv_flows_reserved
where id = :p_app_id; -- Your application ID that is locked
