When someone leaves, there are two steps required:
1- Archive the user
This will free up their license for a new user and prevent them from logging into the system.
2- Revoke their Login
However, if they are still logged in, their credentials will not be checked until they log out and try to log back in. To remove them immediately, their login needs to be revoked.
1. Click on the Cog
2. Click "Team Settings"
3. Click "Manage Team Members"
4. Click the Revoke button against their name and confirm you wish to log the user out. The next time they move to a new page, they will be logged out. If no button is present, they are not logged in using that email.
Learn more: user permissions and security