Every aircraft I've seen has two sets of controls, even aircraft with only two seats. For any number of reasons - we'll go with "This pre-teen is super excited about aviation and wants to fly, but has motor control issues" - a pilot may find themselves with a passenger who can't be trusted to not fuss with those controls.
My understanding is that in anything that isn't fly-by-wire, those controls are mechanically/physically bound together (along with the control surfaces), so I assume there's no cutoff lever that would allow the PIC to positively assert control over the aircraft.
With that understanding, what would the process be to (legally in the U.S., and ideally reversibly) disconnect the controls for one seat?
Obviously each aircraft's mechanical specifics will vary, so the physical part of the job should be summarized but I'm interested to know if such modifications (which presumably don't meaningfully alter the weight and balance) automatically require the aircraft to be registered as Experimental and whether or not this would be mechanically straightforward or a massive undertaking.
EDIT in response to comment:
If a specific plane is required for your answer, assume a Piper J-3 Cub or some other fore-and-aft seater where a rambunctious passenger couldn't just lean over and grab the PIC's controls. If there are specific aircraft where this is easier to effect than others, that's also interesting and should be included in an answer.