How can I put my Nexus 7 back to factory default ?
Have a look at one of these toolkits, they each have an option for installing a stock rom back on to your device. Have included links to osx, windows and linux:


http://forum.xda-developers.com/showthread.php?t=1837401 (mac)
http://forum.xda-developers.com/showthread.php?t=1809195 (pc)
http://forum.xda-developers.com/showthread.php?t=1848036 (linux)