When providers leave your practice you will need to mark them as inactive. Before you do this you will need to make sure that all appointments have been moved across and any treatment claims have been completed.
See: Transfer Patients and/or Appointments to a New Provider (Search and Change)

You can mark a provider as inactive in the provider section by going to File->Providers

To select a provider click on the Select a Provider button:

Highlight the provider the click Ok.

Tick the Not Active checkbox and click Save.

TIP: When you have made a provider inactive, you may need to disable recalls, remove their appointment column and move over any patients and/or appointments. Below are all the guides you need to ensure your providers are removed effectively.

Remove a provider from your Recall Configuration
Add Or Remove Appointment Book Provider Columns
Transfer Patients and/or Appointments to a New Provider (Search and Change)
Configuring Online Booking Appointment Reasons, Services & Providers