InformixClient for Wins+Delphi+ODBC and DB security

I have a client with our old legacy application with UNIX users working on terminal emulations. All database security is in application logic. Now we developed new Win application using Delphi5 for management reporting and GUI presentation of database data ("read only" application). I installed and configured Informix CLI and Delphi BDE and everything is working OK. My problems is: How can I prevent PC users to install ODBC driver, connect to DB using their UNIX user names and passwords (they use them to run application) and use some tool (MS query for example) in which they can write and execute SQL statements DELETE, UPDATE or INSERT? Users must have table level privileges to do that SQLs because they are doing that in application. I just don't want that they can do that outside the application from PC client. If that isn't possible, can I log all connections coming from those clients and executing "destructive" SQLs. UNIX platform is AIX 4.3.3 and Informix is 7.31.UC6. Thanks