BuddyPress.org

You could use this plugin — https://github.com/r-a-y/buddypress-usernames-only — to override the Display name so that only the username shows. I find this makes a lot of sense in communities where @mentions are a prevalent means of connecting. By default the Display name on the profile will still be the Nice Name, rather than the username, but you can change that by adding this snippet to your wp-config.php file

define( 'BP_SHOW_DISPLAYNAME_ON_PROFILE', false );