If a patient is not welcome anymore in your clinic, you can mark them as not welcome in their patient profile. This also prohibits them from making online bookings.

To do so, open the patient profile and click Edit patient.
Scroll down to the bottom and select Not welcome. This will automatically prohibit them from booking online also. 

It's also possible to only prohibit online booking for a specific patient. This way the patient will be directed to call the clinic and the call handling service will be able to direct the patient.