You probably know the sql statement to drop a user
DROP USER pippo;
but you could get this error
ERROR: DROP USER: user "PIPPO" owns objects, cannot be removed
The short answer is: just run this query
SELECT database, objtype, objname FROM _V_OBJ_RELATION_XDB WHERE owner = ^pippo^
which shows you the objects owned by the user you want to drop. You should alter those objects and transfer their ownership to another user.
You can install my Netezza utilities to simplify this task and save time.
Get the list of objects owned by user pippo.
Then connect to every database containing objects owned by pippo and transfer ownership to user pluto.
\c mydatabase CALL util..transfer_objects_owned_by('pippo', 'pluto');
Finally, when no other object is owned by pippo you will be able to drop it.