With the Q2 SP1 2012 release we introduced an easier way to disable dates in RadDateTimePicker control. Now all that is needed to disable certain dates in the calendar of the control is to bind the BlackoutDates property of the control to a collection of dates.
RadDateTimePicker control uses RadCalendar to show its calendar and the approach on using BlackoutDates property in the two controls is the same. For detailed information on how to set up and use BlackoutDates read the help article about BlackoutDates in RadCalendar.
You can download a runnable example project that demonstrates how to use the BlackoutDates, SelectableDateStart and SelectableDateEnd from our online SDK repository here, the example is listed as DateTimePicker / AllowedDates.