Appx main menu > System Administration > Databases/Applications > Applications.

Select the application you want to protect from changes. Press ENTER to go to the popup continuation window. Turn off the flag in the lower right of that window called "Allow Design?".

This makes that application "Inquire Mode Only" from within application design.

Release 4.2 was change to have the default mode in Application Design be Inquire rather than Change mode. Part of the reasoning behind that decision was to assist in protecting applications from accidentally being changed.