If your former employee had a organization phone, you can use the Exchange admin center to wipe and block that device so that all organization data is removed from the device and it can no longer connect to Office 365.
- Go to the .
- In the Office 365 admin center, in the lower-left navigation pane, expand Admin centers and select Exchange.
- In the Exchange admin center, navigate to Recipients > Mailboxes.
- Select the user, and under Mobile Devices, choose View details.
- On the Mobile Device Details page, under Mobile devices, select the mobile device, click the Wipe Data icon , and then click the Block icon.
- Click Save.
For more information on how to secure a former employee's Office 365 data see Overview : Remove an Employee and Secure Data in Office 365