Currently there is a DisabledDates param on your DatePicker component which is extremely useful, however on the DateTimePicker, it appears there is no way to disabled a list of DateTimes. For now it seems the only work around is once a disabled date is selected, to fire an event and inform the user they cannot select that date and null out what they selected. However a very nice feature to have would be to pass in a list of disabled dates directly to the DateTimePicker similar to DatePicker.
Code example:
Possible:
private List<DateTime> _disabledDates = new();
<TelerikDatePicker DisabledDates="_disabledDates">
Not Possible (Requested Feature)
private List<DateTime> _disabledDates = new();
<TelerikDateTimePicker DisabledDates="_disabledDates">