You can archive a Stack by heading to the Settings page then clicking on "Stacks" you then need to hover the mouse over the Stack you'd like to archive until the red archive icon pops up as illustrated below. Click it and then hit "Archive" and it's done.

You can access archived Stacks by going back to the Settings and pressing the on the "Archived Stacks" dropdown.

Your archived Stacks would then show up in your list of Stacks with a trash icon next to them. If you'd also like to restore the Stack then just hover your mouse over the archived Stack and click on the "Restore stack" that pops up as shown below.