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GitHub User: ejholmes
The simplest method is to copy the app/views/active_admin/devise/sessions/new.html.erb file (you can find it here https://github.com/gregbell/active_admin/blob/master/app/views/active_admin/devise/sessions/new.html.erb) to your application directory, then just make any changes that you might need.
Internally, active admin uses devise, so you can go as far as just doing away with active admin's devise config, by replacing the following code in your routes.rb file.
# devise_for :admin_users, ActiveAdmin::Devise.config
And updating config/initializers/active_admin as you see fit.