2 ways -
Log in to another user account that can gain root privledges or boot to another install that either uses the same username or can gain root privledge. Then browse to that file & rename or delete, when you log back in to the account in question you'll get a new user file.
Easier - open system settings > keyboard > layout settings > Options > & enable 'key sequence to kill ..' as in screen
Then go to .config/dconf/ & rename user to a bak. As soon as that is done kill X, re-login
(the point is you want to kill X before anything new is written to dconf
Note: it's possible that with a new user file that you'll get the frickin unity launcher ect. when 1st. logging in to the new user file settings. If that happens go to .local/share & delete the "session_migration-gnome" file, then log out/in