When teams are no longer relevant, they can be deleted by a Organization owner. To do so, click the v icon beside the Organization name in the blue header bar and select "Teams."

Once on the Teams page, click the v next to the team name and select "Edit Team."

In the Edit Team dialog, click the red, "Delete this team" link at the bottom of the window.

This will take you to a confirmation form where you must enter your password before clicking "Delete Team" to confirm the deletion. Deleted teams cannot be recovered so be absolutely sure you want to delete them before doing so.