Doc,
Having some 'issues' with this:

I shutdown the DB, commented out the UNDO_TABLESPACE entry in the pfile and created a new spfile from this pfile.

I restarted the DB - Did a 'show parameter undo' and saw that the undo_tablespace parameter was still set to the old value.

I did an: ALTER SYSTEM SET UNDO_TABLESPACE=''

Then shutdown and restarted using the pfile/spfile which has that parameter commented out and it, again, restarted with the undo_tablespace set to the old value.

How can I set this parameter to '' and restart the db?