In the Office 365 Exchange admin center to follow these steps to block your former employee from accessing their email.
- Go to the Exchange admin center.
- In the Exchange admin center, navigate to Recipients > Mailboxes.
- Double-click the user and go to the Mailbox features page. Under Mobile Devices, select Disable Exchange ActiveSync and Disable OWA for Devices, and answer Yes to both when prompted.
- Under Email Connectivity, select Disable and answer Yes when prompted.