The part(s) or condition(s) listed below for the symptom Garbage disposer stopper switch won't shut off disposer are ordered from most likely to least likely to occur. Check or test each item, starting with the items at the top of the page.
The start switch might be shorted out. Use a multimeter to test the start switch for continuity. If the start switch has continuity all of the time, this indicates that the start switch is shorted out. If the start switch is shorted out, replace it.