Let's say you have a client who has been abusing the online scheduling service, e.g. making too many appointments, or changing appointment times too frequently. You would like to prevent them from using the online scheduling service, but allow your other patients to continue to use these features.
To do this, simply ERASE their login name and password from their patient information. (Changing the login name or password won't work, since they could have that new information emailed to themselves using the "forgotten password" feature.)
Where do you make that change? Display the patient information, and click the "Edit Info" button. Open the section titled "Appointment Reminders and Online Scheduling" and backspace over both their login name and password. Save this by clicking the button at the bottom of that page, and they will be unable to log in from that point on.