Hello, Bart,     
Similar to Outlook, RadScheduler doesn't prevent adding conflicting appointments and this behavior shouldn't be changed. Note that the desired functionality can be easily implemented by handling the AppointmentAdded event and removing the new appointment from the Appointments collection. Here is a sample code snippet:

        private void radScheduler1_AppointmentAdded(object sender, AppointmentAddedEventArgs e)
        {
            if (e.Appointment.Start.DayOfWeek== DayOfWeek.Monday)
            {
                this.radScheduler1.Appointments.Remove(e.Appointment);
            }
        }

If it doesn't suit your scenario, feel free to submit a support ticket providing more details about the precise case. Thus, we would be able to think about a suitable solution and assist you further.