To archive an old team, click on Teams. Click on the edit icon associated with this team in the Current Teams table. On the Edit Team page, you will see a checkbox Yes, archive my team. Click on this checkbox and click Save. Now when you click back on Teams, your old team will be in the Archived Teams table.
If you do not have edit access to the team, you can hide it (or unhide it) on the left-hand navigation by clicking on the visibility icon. A green visibility icon means the team will be visible and a red visibility icon means the team will not be visible.