It depends on the degree of the disability. It they are brain dead, you may exterminate them. If they are only mentally impaired you may not. You might not have any interest in them, but their parents, spouse, relatives, friends could and the State does as it must still protect the rights that that person does still have.If as an objectivist my impediment to using force is the idea the that the violation of another persons autonomous exercise of their will is immoral, what happens when I encounter someone who cannot exercise rationality. Use of force on them is not a moral contradiction, they are not useful to me in any way given that they cannot contribute to society so I have no self-interest in the matter.
But it would be negligent for you to not take an interest in how such people are dealt with for your own sake. At any given time you could suffer an accident that would turn you into a near vegetable. You have an obligation to yourself to support rational definitions of the rights of such people, like yourself if such should befall you.