Can you kill yourself when you close your nose with your hand?

i.e to stop breathing

You would probably start to pass out, your hands would lose their grip and your involuntary breathing system would sustain you. HOWEVER, that would be a very dangerous and very stupid thing to try. Events you didn't anticipate can happen in stunts like that turning it into a tragic outcome.

apart from breathing with your mouth... your survival instinct would kick in and you would let go, voluntarily :)