There is already the possibility to clear out the all the purchased products - in Android please tab the list name and select the last point of the dropdown menu that says "Clear purchased". 

To clear the purchased products off your shopping list on iOS-based device, scroll to the very bottom of your list until you see "Pull to clear" words. Pull the list from bottom to top until "Release to clear" text is displayed. Release the list. 

Hello, you can already do this on our site - http://buymeapie.com/lists