How can I prevent Photos.app from being launched on OS X? I am trying to find a way to either delete Photos.app (or any other built-in for that matter) or stop it from ever launching (until I decide otherwise).

By the way, I have disabled the auto-launch when I connect a device. But I don't want Photos.app to suggest that I should convert my Aperture library by just clicking on one button. If other people are using my Mac this could happen and did before. Then I have to go in and manually edit my precious library again and I am afraid of destroying the hundreds of hours of work that I already put into it.
Maybe creating a separate user account for Others and move /Applications/Photos.app/ to /Users/Others/Applications/Photos.app and change the ownership of that helps you.