In order to block a whole day on your schedule in order for it to not be able available for Self Scheduling on the patient portal, you should add an All-day Appointment with a Task Type of Non Patient Care on each resource that is shown on the portal.

1. Checking which Resources are available to be scheduled from the Patient Portal, see which ones have "Show In Portal" checked:

Scheduler ->Scheduler Options -> Add Resources:

This shows Doctor Smith and One PrescriberOne are both available on the patient portal

2. On each of these resource, Add Appointment, then check "All Day Appointment" and pick the appropriate Non-Patient Care Task Task Type and Save