Informix security

We use MS-Access 97 as a front end to the Informix Online Dynamic Server V.7.23 running on RS/6000 and AIX V4.3. We link the tables via ODBC. When we create the links, we have to supply a valid user-id/password, or the link fails. To complicate matters, we have automated the startup of the Access 97 database by supplying the password in an autoexec macro - otherwise the first time the tables are accessed in any way, ODBC asks for a password. All users are using the same password, but most of them don't even know it. By linking tables, we have found that some tables can be updated through the Access front end, and some cannot. I know the security on the Access links are the same for both types of tables, yet users can update one table, but not the other. The dbschema for Informix looks the same for both tables also. Why is this allowed? We would like it so that NONE of the informix tables can be updated through ODBC/Access 97. We have another software application that is writted for the database that does allow updates. What is the suggested informix security configuration for this setup?