You really should ask the plugin authors this question using their support forum:
https://wordpress.org/support/plugin/bp-registration-options
I’ve never used their plugin, but do you have ‘moderation’ turned on?
From their readme file:
* Hide “Person became a registered member” from activity stream if moderation is turned on until person is approved.
This will prevent new registrations from showing in site activity, regardless of whether you’re using their plugin… but maybe you need something else?
remove_action( 'bp_core_activated_user', 'bp_core_new_user_activity' );
