The best way is to have an application field in with a checkbox “No, I am NOT able to attend this event” in the registration form. Also make sure you have not enabled auto confirmation applications.
When registering for the event, the would be attendee has the option to indicate that she will NOT be able to attend the event.
When processing event applications, set registrations that have indicated that they will attend as “confirmed”, for others set their status to “turned down”.
There is no functionality to automatically mark applications as “turned down” based on the response to such a question.