Journaling is required for a table to be updated/deleted in AS400. Effectively if journaling is off for the table it is read-only through DB2.
One other problem I've run across when updating to the AS400 had to do with the setup of the ODBC driver itself. For the iSeries ODBC driver verision 9.00.00.00 there are settings that can cause diffculty and seem to work for updating or selection, but not both. In the Performance tab, under the Advanced button, these are the settings:
Open All Cursors as updateable --Set ON for update, Off for select
Retrieve extended column information --set ON for update, Off for select
I've had to set up 2 different datasources, one for read, and another for write. If anyone else has run into this and solved it, or if there's a version up that's fixed this I'd love to know.