Hope to get help or advise on my issue as below.

My in-house application system is developed in 1990 using SQL/ACCELL programming language and it sits on Unify database version 8.2 running on Linux Redhat.
My users use Putty software to act as terminals to access to this in-house application system. However, some of users has accidentally clicked the close button of putty software to logout from the system that resulted our Unify database slow or hang due to the irregular logout.

Pls advise how to disallow my users to click on the close button or other alternatives to overcome my issue.

Thank you.

If the users are using PuTTY on Windows, this isn't really a Linux question. The close button is part of the Windows OS, you're not really going to be able to disable it.

If your users, once logged in, use screen, then connect to the database, their database session will keep running should the user close PuTTY/their computer without logging out. It may be possible to automatically use screen when the user connects. Of course, you must decide whether having users left logged into the database (possibly for several days) if they don't log out properly is better than the current situation.

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Thanks for the advise from cantab. Pls suggest any available softwares that runs on Windows (besides Putty) that can connect to Linux that can overcome my issue.

I don't think you will find a Windows app without a close button; as said before it's part of the standard OS window.