If you want to clean out schema you have to First disable constraints and then drop the objects. Use the following script and login as that schema owner and try running the script.

Easyway is 'drop user xyz CASCADE' and recreate the user. Do this only when you want to remove everything from the schema. Otherwise control objects removal from script.

Another option is drop the data of the tables by using truncate.
and then drop the objects, if you don't want to disable constraints and then drop.

<font face=courier>
prompt -- !! WARNING !!
prompt --
Prompt -- Don't run script as SYS or SYSTEM.
Prompt --
prompt -- !! WARNING !!

set echo off;
set verify off;
set feedback off;
set pagesize 0;
set head off;

spool druserobjects.sql

select 'drop '||object_type||' '||owner||'.'||object_name||';'
--from dba_objects
--from all_objects
--from user_objects
where object_type in ('INDEX','PACKAGE','SEQUENCE','SYNONYM','TABLE',
'TRIGGER','VIEW','PROCEDURE','FUNCTION')
and owner in
('XXXXXX','YYYYYY') ;
SPOOL OFF;
SET VERIFY ON;
SET FEEDBACK ON;
</font>