Only members with the capability ‘bp_moderate’ will see that checkbox option.
And only site administrators are given that capability.
So unless you have a plugin that gives that capability to other members, nobody else can see that option. To check, log into your site as a non-admin.
If you still want to remove it, create a template overload of this file:
buddypress\bp-templates\bp-legacy\buddypress\members\single\messages\compose.php
In your overload, remove this:
<?php if ( bp_current_user_can( 'bp_moderate' ) ) : ?>
<p><label for="send-notice"><input type="checkbox" id="send-notice" name="send-notice" value="1" /> <?php _e( "This is a notice to all users.", 'buddypress' ); ?></label></p>
<?php endif; ?>
