i tryied to drop a tablespace called "TOOLS" - no broblem, just wrote :
DROP TABLESPACE TOOLS INCLUDING CONTENTS AND DATAFILES;
worked just fine. then i tryied the same for the tablespace XDB and it didnt worked. so, i started to think that the XDB tbs is somehow more problematic to mannage (like SYSTEM tbs or somthing).
BUT Then You Came along and you were absolutly RIGHT !
I Dropped the user XDB and then the tbs :
DROP USER XDB CASCADE;
(Cascade - Drops all the user Objects as well.)
and then :
DROP TABLESPACE XDB INCLUDING CONTENTS AND DATAFILES;