You still did not answer my original question. How are you executing the application program when the user logs in? Are you putting the name of the executable program in the .profile file so that it runs when the user logs in. The answer to this question will get you a solution. SCO login is secure. If you are breaking into the system with the delete key, it is most likely due to what is being done through the login process as far as executing your program. To test this, try putting a password on the account and see if you can break in at the login prompt. This will give us more information. You can remove the password after the test.
It is not a good idea for multiple users to use the same login or for that login to not have a password. In the first case, process limits can be reached more quickly and in the second case, security is an issue.
In general, you will find this forum is very helpful when we fully understand exactly what the problem is so hang in there.