Apologies for the thread necromancy, but I recently had to do exactly this. This old code no longer works, as the structure of the appointments form has changed. I have some updated code that does work, at least with v18.1.

The advantage of this code is, with come modifications to the FindControl function to search by name, you can use it to modify other parts of the form, like hiding the "Location" dropdown.

This is vb.net code, but you can translate it for C#.

    Private Sub UltraCalendarInfo1_BeforeDisplayAppointmentDialog(sender As Object, e As DisplayAppointmentDialogEventArgs) Handles UltraCalendarInfo1.BeforeDisplayAppointmentDialog
        Dim uciCalendar As UltraCalendarInfo = DirectCast(sender, UltraCalendarInfo)
        Dim utcTabControl As UltraWinTabControl.UltraTabControl

        e.Cancel = True

        Using adDialog As New AppointmentDialog(uciCalendar, e.Appointment, e.IsExistingAppointment)
            utcTabControl = FindControl(Of UltraWinTabControl.UltraTabControl)(adDialog)
            If utcTabControl IsNot Nothing AndAlso utcTabControl.Tabs.Exists("Action") Then
                utcTabControl.Tabs("Action").Visible = False
            End If

            adDialog.ShowDialog(Me)
        End Using
    End Sub

    Private Function FindControl(Of T As Control)(cParentControl As Control) As T
        Dim cFoundControl As Control = Nothing

        If cParentControl Is Nothing OrElse cParentControl.Controls Is Nothing OrElse cParentControl.Controls.Count = 0 Then
            Return Nothing
        End If

        For Each cControl As Control In cParentControl.Controls
            If cControl.GetType().Equals(GetType(T)) Then
                cFoundControl = cControl
                Exit For
            Else
                cFoundControl = FindControl(Of T)(cControl)

                If cFoundControl IsNot Nothing Then
                    Exit For
                End If
            End If
        Next

        Return cFoundControl
    End Function