I'm not sure what you mean with disactivate...
You have to imagine every object that can interact with events (suck as mouse click, key stroke, etc) like something that use the event and then "consume" it.
Once consumed, the event generated is destroyed.
So basically: one event, one use.
Since there are many things that can consume an event (e.g buttons, or other pieces of code), Godot gives you an order on which objects are checked for consumption (see the list in the documentation i shared).
_input(event) is the higher level, which means that if the function inside intercept the mouse click, it will not be used by anything else, not even buttons.
Unhanlded input is the lower, it takes events that have not being used by anything else (including buttons), and check if they can still be of use.
Hope it is clearer now