Stopping a crewmember from going crazy would hopefully be prevented before they went to space. NASA was exceedingly strict about who they let on their rockets, such as Ken Mattingly, who was supposed to be on Apollo 13, until exposure to German measles kept him from going. He never contracted said measles, NASA stopped him because he was just exposed to it through another astronaut, Charles Duke. NASA also did psychological testing and had many stringent requirements: A bachelor's degree or similar in engineering, hundreds of hours of flying time, good physical condition, and not being too tall (modules were cramped). This rigor shows in the astronaut's actions, and the audio recordings of Apollo 13 show how well disciplined and calm these men were in extreme crisis. Chances of a pilot going crazy were lower than one might think. Funny thing about Apollo and aliens, when the first mission came back, the boys of Apollo 11 were held in quarantine in fear of some sort of "moon germs", sounds a little silly, I know, but they were for all intents and purposes coming from a complete unknown, so they were quarantined for 3 weeks. Not the aliens you'd think of, and although it was considered a remote possibility, it shows the effort NASA put towards these missions.

As for the moon missions, they were intended to splashdown in water. Why is this important, you may ask? They splash into the water and some US carrier comes to pick them up. However, for the Soviets, their capsules (except for one misadventure involving crashing through a frozen lake, Soyuz 23) never had splashdowns, and always landed on good, Russian soil. What's the issue? Good Russian soil includes the Siberian wilderness, with wild animals like bears and wolves. After Vokshod 2 landed in Siberia, the cosmonaut Alexei Leonov expressed concerns about the 9 mm pistol he had in his kit not being enough to hold off bears and wolves. The result was the magnificent TP-82 Cosmonaut Survival Pistol, a triple barrel weapon that can fire shotgun or rifle ammonution (imagine a double barrel shotgun with a rifle barrel beneath that. Can't? Here's a [picture] (https://en.wikipedia.org/wiki/TP-82_Cosmonaut_survival_pistol#/media/File:TP-82.jpg0). Please don't kill me for the wikipedia link, wanted an image.) and it also could launch flares and the buttstock was also a machete. I believe the Soyuz capsules now use semi-automatic pistols, since the ammunition has stopped production.

edit: Also, firing guns in space is a fairly bad idea. The recoil would send you flying (or at least push you) backwards after firing, maybe even sending you into a spin. Not to mention why firing a bullet in space is a tremendously bad idea, as guns were adopted primarily for their ability to pierce metal, and a spaceship is made of metal. If you wanted to stop a rogue crew member, I would advise a knife unless you're up to adding a new hole in your ship.