SQL & PL/SQL :: Automate Locking / Unlocking Of User Accounts

I have been plagued by people logging into my database and making changes when a clone is in process.. Having said that ,I am looking to lock accounts and unlock them when I am done.

I envision my code looking something like this:

sqlplus -s / <<END
SET PAGESIZE 0
SET FEEDBACK OFF
SET VERIFY OFF;
set heading off;
spool /tmp/lockusers.sql
select 'alter user ' || username || ' account lock;' from dba_users where username not in (....) and not locked?;
spool off;
END;

sqlplus -s / <<END
@/tmp/lockusers.sql
END;

When it comes time to unlock the accounts I want to be able to unlock those accounts I previously locked and not all of them. Is there a query, I can use that can tell me when the accounts were locked or some other way about going about this so I dont unlock accounts that were locked prior to my lock script running?