Stop XQuartz from showing icon in the OSX dock and stop loading xterm on start

Edit /Applications/Utilities/XQuartz.app/Contents/Info.plist as root (ie, sudo vim in a terminal), and add the following key/value inside of <dict>:

<key>NSUIElement</key>
<string>1</string>

…. and in your terminal run defaults write org.macosforge.xquartz.X11 app_to_run /usr/bin/true, and then restart XQuartz: the dock icon should be gone and it shouldn’t start xterm anymore.