But this goes back to the problem of whether the person is even able to comply, or aware of the skymarshall's orders to begin with. It assumes that the person is still in control of their actions (not always the case with mental disorders) or completely aware of their surroundings (ditto; also see Shera's point about people with hearing problems).
If you cannot understand, or comply with, the instructions of the aircrew/police etc then you should not be in a secure area. (i.e. a plane).
Take a bus/train/ship instead.