I'm going to assume that you tried the Nanoclock uninstall procedure by setting the uninstall option in the [global] section of the config file to true and it didn't work.
To do a true factory reset, you can use the developer menu to do a real factory reset or use the manual method. See
Manual reset your Kobo Sage, Kobo Libra 2, Kobo Forma or Kobo Libra H2O on Kobo's help pages for more information.
The setting menu factory reset is about 1 step above signing out/signing in.