How to Modify/Change a Buddypress/WordPress Account Activation Process

You can add the bp_core_signup_send_activation_key filter to prevent the activation email being sent as follows

add_filter('bp_core_signup_send_activation_key', 'my_bp_core_signup_send_activation_key', 10, 5);

function my_bp_core_signup_send_activation_key($send, $user_id, $user_email, $activation_key, $usermeta) {
    return false;
}