I forget it over and over again, now its here archived :-). The following statement generates DROP TABLE statements which really cleanup the Oracle database:
select 'drop table '||table_name||' CASCADE CONSTRAINTS;' from USER_TABLES
It is very useful for development (JPA will generate the tables for you again), and production (speeds up the performance) :-).
Nice thing: USER_TABLES and Oracle's dictionary is conceptually similar to reflection in Java.