Hi John,
Thank you for your interest in Syncfusion Products.
You can cancel the client-side events by returning false value. Please refer below online sample which illustrates how to cancel ClientSideOnAppointmentDrop event:
http://samples.syncfusion.com/ASPNET/7.2.0.37/Web/Schedule.Web/samples/3.5/Scheduler-BasicFeatures/DragDropOptions/cs/DragDropOptions.aspxWe have provided detailed information of ScheduleAppointmentEventArgs of each client-side event in below link:
http://help.syncfusion.com/ug_72/ScheduleWeb/Client-sideEvents.htmlPlease let me know if you need any more details.
Thanks,
Melba