Database security from PC applications.

I have been recently tasked with looking into users making changes to our
AS/400 tables from PC applications and how to prevent this.

I seem to recall that there might have been some discussions of this in the
past.

I know that the usual way to do something like this is to create all
database tables with a db owner and exclude *PUBLIC from any access. Then
when the user starts a program the system grants them access to the
application for the duration of the session and when the session ends the
authority is revoked.

The problem is that requires everything to be set up that way and major
application work which they are not willing to do given the AS/400 is toast
in a year.

Any other ways people have accomplished this? I believe there is a database
exit program. Any other ideas?

Thanks for any help.