It's hard to answer this question without more information, but I'll give it a shot.
My first approach would be to write a CiviCRM extension using hook_civicrm_validateForm. This would give a result similar to if someone left out a required field - it would reload the page with an error.
If your goal is instead to allow them to register (and it's a free event) but not see the results, Michael M's suggestion of a CiviRule to delete the event after it's created may work.
If your goal is something else - e.g. to fake a successful event - you'd have to give more details, but is likely to be more difficult.