Account owners manage the team in Settings → Team.
Remove a person: select the remove button next to their name. They lose access to this workspace straight away. Their other workspaces aren't affected.
Delete a pending invitation: select delete next to it under Pending invitations. The link in their email stops working.
Tidy up: select Clear to remove invitations that have already been accepted.
You can't remove yourself. Ask another account owner to do it.