BuddyPress.org

Create a template over-ride for this file:
\buddypress\bp-templates\bp-legacy\buddypress\members\single\messages\compose.php

Then delete or comment out this:

<?php if ( bp_current_user_can( 'bp_moderate' ) ) : ?>
		<input type="checkbox" id="send-notice" name="send-notice" value="1" /> <?php _e( "This is a notice to all users.", "buddypress" ); ?>
<?php endif; ?>