1) We don't have an error message "table locked" -- a delete would wait for rows to become unlocked to proceed. I'd really need more information to help you out such as the exact error you are getting and the environment in which you get it (sqlplus, custom app, forms, etc etc etc)

2) select a.username us, a.osuser os, a.sid, a.serial#
from v$session a, v$locked_object b, dba_objects c
where upper(c.object_name) = upper('&tbl_name')
and b.object_id = c.object_id
and a.sid = b.session_id;

will show you who has locks on the object

3) alter system kill session 'sid,serial#'

where you get the sid and serial# from v$session.