You can deactivate a user once they've been added to your account. Deactivating removes their access without deleting them, so you can re-enable them later. (You can't deactivate your own login.)
Open the account menu in the top-right corner, choose Account Settings, then go to Organization → Users.
Use the filter tabs — All, Active, Disabled, Unverified — or the search box to find the person. Click their row to open their profile, then click the red Disable Access button.
To restore someone, click the Disabled tab to list deactivated users, open the one you want, and click Enable Access.