According to the CBS report:
It turns out NASA has a detailed set of written procedures for dealing with a suicidal or psychotic astronaut in space. The documents, obtained this week by The Associated Press, say the astronaut's crewmates should bind his wrists and ankles with duct tape, tie him down with a bungee cord, and inject him with tranquilizers if necessary.
One big caveat? No guns of any kind—stun or otherwise—are allowed in space. This means the crew is largely dependent on their own strength to control an unmanageable astronaut. Once restrained, the astronaut can be administered tranquilizers or anti-depression, anti-anxiety, and antipsychotic medication depending on their condition. Should they not voluntarily take these medicines, the drugs could be forcibly administered with a shot to the arm.