Allow Self-Service Booking but Block Changes 48 Hours Before Appointment

I am a therapist and my clients can book their own appointments through Calendly, which works great. However, some clients need to cancel or reschedule their appointments. My cancellation policy states that appointments can be changed free of charge up to 48 hours before the scheduled time.

I would like a feature that prevents clients from cancelling or rescheduling within 48 hours of their appointment, while still allowing them to manage their bookings themselves up until that moment.