Lockdown Username Change

class ApplicationController < ActionController::Base
    before_action :configure_permitted_parameters, if: :devise_controller?

    protected

    def configure_permitted_parameters
        added_attrs = [:username, :email, :password, :password_confirmation, :remember_me]
        lockdown = [:email, :password, :password_confirmation, :remember_me]
        devise_parameter_sanitizer.permit :sign_up, keys: added_attrs
        #You remove the :username this way it won't accept it when you change it.
        devise_parameter_sanitizer.permit :account_update, keys: lockdown
    end

Change registrations/edit.html.erb

<%= f.label :username %><br />
<%= f.text_field :username, autofocus: true, class: 'form-control', :readonly => true %>

Congratulations Users can't change username now