How to uncheck "Prevent the user from clicking on the other layers"

Wow. The checkbox should maintain whatever state (selected or not selected) it is in when you close the Slide Layer Properties. 

That slide may be corrupted. You could try creating a new slide to see if that problem still happens. 

If the problem continues, I suggest you contact the Articulate staff directly. You can do that here: Contact Support for Help with Any Articulate E-Learning Course Development Software