After you've saved and accessed all the former employee's user data, you can delete the former employee's account.
- Don't delete the account if you've set up email forwarding or converted it to a shared mailbox. Both need the account to anchor the forwarding or shared mailbox.
In the Admin center, select Users.
Select the employee that you want to delete, and then choose Delete user in the user pane and then choose Delete > Close.
When you delete a user, the account becomes inactive for approximately 30 days. You have until then to restore the account before it is permanently deleted.
For more information, please visit https://support.office.com/en-us/article/remove-a-former-employee-from-office-365-44d96212-4d90-4027-9aa9-a95eddb367d1