Asana Delete Unused Teams

The Team Management window:

Once you have left-clicked on a team, you should see the "Overview" page for that team. 

Navigate to the drop-down arrow next to the team name, and left-click "Edit team settings"

Copy the name of the team here for later.

Deleting the team:

In the new overlay that popped up, navigate to the "Advanced" tab along the top row.

At the bottom of the "Advanced" tab, should be the option to "Delete team..."

Left-click "Delete team..."

The point of no return:

A warning will be displayed informing you that deletion of this team will delete all projects, templates, and tasks associetd with it, this is non-reversable. Enter the name of the team, as it appears in top left of the overview page into the  text box, and click "Delete Team"

Once you've cleaned up your teams, clean up some of those old projects!