It's a libnickel patch. Look for
Code:
<Patch>
patch_name = `Block WiFi firmware upgrade`
patch_enable = `no`
# Multi-version patch: 3.3.0 - 4.4.9298+
#
## WARNING: THIS IS A DANGEROUS PATCH! READ THE INFORMATION BELOW CAREFULLY!
##
## THIS PATCH IS UNTESTED, as there is no way to be sure it still works until
## the next firmware version is released. If it works then the firmware will
## not be upgraded during a WiFi sync, but you will still be able to upgrade
## manually or by syncing with the Kobo desktop.
##
## THIS PATCH COULD RESULT IN A BOOT LOOP when signing out of your Kobo account
## (Settings > Accounts > Sign out) or when invoking a factory reset from the
## device information menu (Settings > Device information > Factory reset).
## REMOVE THIS PATCH BEFORE USING EITHER OF THOSE OPTIONS.
## A factory reset invoked using the hardware methods (such as holding down the
## light button while switching on) should not be affected.