I believe I found it within /controllers/system.php
CNotificationLibrary::add('profile_activity_add_comment', $my->id, $commenters, JText::sprintf('COM_COMMUNITY_ACTIVITY_WALL_PARTICIPANT_EMAIL_SUBJECT'), '', 'profile.activityreply', $params);
I just commented this out, although a sophisticated approach is needed. The emails served a purpose of bringing them back to the conversation, so removing them completely is not good. It would be nice to have a system that emailed periodically, like LinkedIn.