function victor_dont_save_join_group_activity( $activity_object ) {
 
    $exclude = array( 'joined_group' );
 
    if( in_array( $activity_object->type, $exclude ) )
        $activity_object->type = false;
 
}
add_action('bp_activity_before_save', 'victor_dont_save_join_group_activity', 1, 1 );