[_ck_]

It was also reported here:

https://bbpress.org/forums/topic/gravatar-disable-issue

There is a bug in bb-admin/options-general.php

https://trac.bbpress.org/changeset/1499

Edit bb-admin/options-general.php

and right near the top after

bb_check_admin_referer( 'options-general-update' );

add this line:

if (empty($_POST['avatars_show'])) {$_POST['avatars_show'] = false;}