How ( and mostly at what time ) can i prevent the alternate cron from running?

I’m using the alternate cron version, but i need to prevent it from running on some calls, how can i do this?

In the cron.php file i’ve seen on line 225

if ( defined('ALTERNATE_WP_CRON') && ALTERNATE_WP_CRON ) {
    if ( !empty($_POST) || defined('DOING_AJAX') )
        return;

but i’ve tried defining define('DOING_AJAX', true) and it never stops the cron. Maybe i’m defining the constant too late. When should i define it?