How can I delete sprints from before I converted to next-gen?

[Nik]

When I converted to next-gen, I had a couple sprints planned out, but not active. After the conversion, I no longer see the sprints in the backlog, but I do see them as options in the Sprint field as future sprints. Additionally, on my reports page, I have what appears to be a multitude of very unproductive sprints, as I foolishly elected only to transfer uncompleted tasks, so all my completed sprints have zero tasks in them. Is there a way to delete these sprints so they will stop cluttering up this otherwise great UI?