How to get rid of the cross in the upper right corner of the dialog?

I created a dialog and put there the button labeled “Exit” and connected some macros to it. This button ensures the correct exit from the dialog. But sometimes my inattentive friend closes the dialog by clicking the cross in the upper right corner of the dialog. How can I get rid of this cross?

Hello,

Take a look at this post → Prevent a dialog to close with the x button