Comment # 3 on bug 964562 from Hot MusicFan

(In reply to Takashi Iwai from comment #2)
> The symptom sounds different from boo#955096.  That bug was about the
> blockage of shutdown/reboot while yours is spurious wakeup after shutdown. 
> This is mostly due to either ACPI or XHCI wakeup.
> 
> Check /proc/acpi/wakeup.  For each "enabled" item, echo the device name,
> e.g. if "USB1" shows "enabled", run like
>    echo -n USB1 > /proc/acpi/wakeup
> Then this item will be disabled.  After disabling all, try to shutdown and
> check whether the reboot happens.
> 
> Disabling the ACPI wakeup would also disable the wake up after suspend, of
> course.  It's just for figuring out the cause.
> 
> Another possible spurious wakeup source is USB.  XHCI driver has various
> workarounds for such firmware problems.  You can pass quirks option to
> xhci-hcd module, e.g. try to pass "xhci_hcd.quirks=0x42000" boot option.

I have disabled all enabled processes in /proc/acpi/wakeup as you suggested but
there is just no result. The laptop reboots everytime independently if it is
running on battery or on power supply; Also I have entirely disabled Wake on
USB from UEFI.