The part(s) or condition(s) listed below for the symptom String trimmer head won't stop spinning 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 clutch engages the clutch pads to spin the trimmer head. If the clutch assembly is defective, the clutch will not disengage the clutch pads, and the trimmer head will continue to spin, even at low speeds. If the clutch is defective, replace it. (Note: some string trimmers do not have a clutch. If your string trimmer does not have a clutch, the trimmer head will spin whenever the engine is running.)