You should place this button in the Template set in EditAppointmentDialogStyle, because the DataContext of the TargetType of this style is AppointmentDialogViewModel and the command is defined there. You can edit and remove the needed controls in the template.
In order to customize the EditRecurrenceDialog, you should set EditRecurrenceDialogStyle property of the ScheduleView. I've modified the project to demonstrate the approach, please download it again and examine it.
Hope this helps.
All the best,
the Telerik team
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