This gem activeadmin-wysihtml5 (available at https://github.com/stefanoverna/activeadmin-wysihtml5) from Stefano Verna gives you a simple way to add a WYSIWYG interface to ActiveAdmin inputs.
But I was needing just the WYSIWYG part, without the image upload feature. In this case, the solution was just not add the image item in the commands list in ActiveAdmin's configuration.
ActiveAdmin.register ModelName do
form do |f|
commands: %i(bold italic underline ul ol outdent indent link source),
Since activeadmin-wysihtml5 registers a new model - called Asset -, a undesired link was keeped at the top of administration interface. To hide it:
ActiveAdmin.register Asset do