If you're subscribed to a Team plan, you can add and remove Teammates through the Team admin section.
Access this section by clicking the "Team" icon on your Project Dashboard.
Find the user you'd like to remove and click on their name to expand the panel.
If you'd like to remove the user from your Team, click the "Remove from team" link.
You can add them again at any time by clicking the "View archived teammates" link at the bottom of the page, then clicking "Restore to Team".