If a user is no longer associated with your organisation, you can easily unlink him/her:
- At the top of the page, click People
- Click View People
- Click Search to bring the data up to date
- Locate the person you want to remove (you can filter on the right-hand side)
- Click the Remove from Organisation button (the X)
Note: This person will now be unlinked from your organisation, but will still have access to their personal account and their email address will still be in use. If you've unlinked someone in error, you can easily re-link the person the same way you would create a new person (instructions here).