Is there still no easier way?

it is OK, thanks for help.

The ClickToAddActivityElement has nothing to do with the dialog; that simply relates to the "Click to Add Appointment" prompt that is displayed within a timeslot when you hover over it. If you don't want users to add using the activity dialog (i.e. what you see when you double click on a timeslot) then you can handle the ActivityDialogDisplaying event and set e.Cancel to true as needed (e.g. if the DataItem is null). If you are using 11.1 then you can set the IsAddViaDoubleClickEnabled on the ScheduleSettings->AppointmentSettings to false.

I implement to add some Appoiments by hide code.  For dont want users add Appoiments by 

Dialog I  add:

<Style TargetType="igPrim:ClickToAddActivityElement">
 <Setter Property="Opacity" Value="0" />
 <Setter Property="IsHitTestVisible" Value="False" />
</Style>

So I see that, it still allow user add Appoiments by Dialog.

I'm sorry but I don't understand the issue you are having. Can you provide a sample that demonstrates the issue and steps when running the sample? Or perhaps provide a more detailed description with screenshots?