I want to unschedule an event but keep the original in the Item List

You can unschedule or remove events from the calendar but keep your original in the Item List.  How you do this will depend on the type of event that has been scheduled.

To unschedule recurring events (multiple events in a series like meetings). Locate the event in the calendar and choose Delete or hit the DEL key

To unschedule a single event.  Locate the event in the calendar and drag it back into the Item List and drop.