When double booking for therapists is set to warn, you can still create an appointment over a slot with an existing blockout time. On booking the appointment, you will get a warning message saying that the therapist is already booked.
To avoid overlapping blockout time and appointments, double booking for therapists must be set to block.
To block double booking for therapists
At the organization level, go to Admin > Organization > Organizations.
The Edit Organization page opens.
Go to the Settings tab.
Expand the Appointment Book section.
Set the Double-booking for Therapists option to Block.