At this moment, you can only drop the events on another Scheduler instance.
There are two ways to approach this:
1. Use a context menu item (something like "Unschedule") to remove an event from the Scheduler.
2. You can create a special instance of Scheduler to implement the queue of unscheduled events. If you configure it to display just one cell you can use it to display a simple list. Note that dragOutAllowed property needs to be set on both instances (https://api.daypilot.org/daypilot-scheduler-dragoutallowed/).