If you need to disable access to an account, one method is to block the account from signing in. To do this:
Sign in to https://admin.microsoft.com using an Admin account.
Select Users > Active Users. Select the employee that you want to block
Click Edit next to Sign-in Status > then choose Block the user from signing in > Click Save
If you block a user from having sign-in access to Office 365, it might take as long as 24 hours to take effect on all that user’s devices and clients.
We also recommend resetting the users password and signing them out of all apps and sessions