Put this code in a Plugin. The group “Member” will be affected.
function restrict_bb_got_roles() {
global $bb_roles;
unset($bb_roles->roles);
unset($bb_roles->roles);
unset($bb_roles->role_objects->capabilities);
unset($bb_roles->role_objects->capabilities);
}
add_action(‘bb_got_roles’, ‘restrict_bb_got_roles’);