How do I prevent X11 opening an xterm when it starts?
defaults write org.macosforge.xquartz.X11 app_to_run /usr/bin/true


in a Terminal. However, note that it normally shouldn't be necessary to ever start XQuartz.app (or X11.app; see the comments) manually. It will start automatically when you run an X11 client, and then it will not start its defaults startup application.