It's not possible to achieve this smoothly at the moment.
- You are able to handle this on the client side (TimeRangeSelectedHandling="JavaScript") or on the server side (TimeRangeSelectedHandling="CallBack" or "PostBack"). In the event handler, you can deny requests that include the forbidden times.
- However, it's not possible to prevent this before the TimeRangeSelected event is fired (i.e. you will always be able select any cell).