It is in the template files form_reply.php and form_topic.php
The line that calls the WP_Editor class to create the textarea using the relatively new WordPress core front end editor is this one:
bbp_the_content( array('context' => 'reply')
You can look into the bbp_the_content function to see how BBP does it and how you can circumvent default behaviour.
Notice that this line only applies if the fancy editor is enabled for the current user*. Otherwise the template file falls back to display the classical textarea (which is also the one used in this forum).
*) It seems there’s a bug in BBP 2.2.4 making the fancy editor available only to the keymaster. This can be overridden in the code of course.