It would be nice if we had a property setting we could dynamically change like you can in some systems, but FileMaker doesn't come with that built in capability. We can, however, fake it pretty close with a bit of creativity.
Conditional formatting can change the appearance of the object's text to a grey color to signal the user that it is disabled. But then the button's script or the field's script trigger performs a script to determine whether or not anything happens when the user clicks that button or field. Clicking or tabbing into a field will trip the onObjectEnter trigger and the script it performs can use go to field, go to next field, or go to object to put the focus on another layout object to deny access to the field selectively. A button may be "disabled" by enclosing the button's entire script iniside an if block so if conditions indicate that the button is disabled, nothing happens when it is clicked.
Thanks a lot for your help. Very good answer that allowed me solve an important problem.