There is a good reason why such an option does not exist. Just go back to reviewing and relearning as needed. I can personally attest that a stack of over 700 expired is nothing short of 1 Saturday's work.
If you really have to start over (which you shouldn't) you can always create a new account. It may also be possible to politely ask the admin to erase your progress, but weather he does is different matter.
If you want to rest your flashcards, you delete cards from the Manage tab. If you don't want to check several hundreds flashcards manually (Remove Cards > Select from list), you enter a custom range like "1-200" (Remove Custom Selection).
On the website, "learned" has a different meaning. If you click the "learned" button from the Study page, the failed kanji goes to a "learned" pile which is listed on the left sidebar. It is a way to mark failed kanji as "relearned". It goes into a "learned" list (under Restudy kanji, sidebar), which let you do one extra review. Those that pass go to stack 2, those that fail stay in the learned list until you pass them or you reset the list. You CAN reset that list, the red "clear" link should be pretty obvious.