To stop selling a publication on Issuu — in other words, take it off sale — go to your Publication List and click SELL SETTINGS. Scroll to the bottom of the page and click Stop selling (located under Take off sale).
The publication will then automatically revert to its original state (unlisted, public, etc.).
NOTE: Anyone who has already purchased your publication will still be able to access their copy from the My Purchases section in their Issuu account.