This post is older than a year. Consider some information might not be accurate anymore.
To purge all data from a user schema, I simply generate a drop table statement for each user table.
select 'drop table '||table_name||' cascade constraints;' from user_tables;
This will generate something like this:
drop table CASELOCK cascade constraints;
drop table CLIENTS cascade constraints;
drop table CLOSECODE cascade constraints;
..