Cleanly uninstall ubuntu from Nexus 7

Solution 1:

Installing Ubuntu on Nexus7 does not require rooting the phone. It does require unlocking the boot loader which will void your warranty as stated on the device when you unlock it. You can re-lock it later, but unfortunately I don't know if there are any counters that that will be able to use to detect you've tampered with it.

EDIT: chris-wayne who is on my team at Canonical called Google and they stated on the phone that unlocking your bootloader, installing something, then reflashing back to stock and re-locking will NOT void your warranty. We unfortunately don't have this in writing, but it is encouraging to hear.

Solution 2:

I have been assured from Google that as long as you do the following, the warranty will be honored. Flash stock android back on the device (as described in the wiki at http://wiki.ubuntu.com/Nexus7/Installation), and re-lock the bootloader, by placing the device in fastboot mode, plugging into your computer, and running fastboot oem lock

There should be zero issues at this point, it will be as if you just bought a new device.