Drupal 7 incorporated in its modules the code that were in Poormanscron, whose purpose is exactly to run Drupal cron tasks on a site where it is not possible to set cron tasks, when a user visits your site, and every X hours (as set in the page you saw).
If you want to avoid that behavior, you just need to set "Run cron every" to "Never"; contrary to what it could seem, that just influences what happens when users visit your site. You can still run Drupal cron tasks from outside Drupal, remembering that in Drupal 7 cron.php needs to be invoked, from outside Drupal, passing the value shown in "Administration > Reports > Status report."
: In fact, neither cron.php, nor drupal_cron_run() checks the Drupal variable used in that settings page. The only Drupal variables used are:
- "cron_key" that contains the cron key used to avoid the Drupal cron tasks are ran from unauthorized users
- "maintenance_mode" that is set to
TRUE when the site is in maintenance mode
- "cron_last" that contains the timestamp of when cron tasks were ran last time