Apple Reminders.app -- Delete Completed Reminders

gistfile1.applescript

set the theList to {"Reminders", "Groceries"} -- Edit/Add all Reminder lists to remove completed items from

repeat with eachList in theList
	
	tell application "Reminders"
		-- Loop thru reminders in the targeted reminder.app list
		tell list eachList
			set theTasks to every reminder
			repeat with theTask in theTasks
				if theTask is completed then
					delete theTask
				end if
			end repeat
		end tell
	end tell
end repeat