There's no reset button, but you can do the following:
1) Re-apply a theme, any theme, from the Gallery page (under Assets). This will do away with the theme you have now. (I sure would download my theme before doing this, as backup. Seems extreme.)
2) Delete all your products
3) Delete all your collections
4) Delete extra link lists you've created
5) Delete all pages except About us and Frontpage.
6) Delete blog posts.
Visit your preferences pages and remove extra coding, etc.
You can also just sign up for a new account, that would give you a 30-day free trial with a blank slate. You won't be able to use the same myshopify domain in the other account though.
I have the same question. I can't simply uninstall all products collections etc. When I do that and start working again, my theme is still missing the Home and Catalog and Search pages. It seems I need to reset EVERYTHING. Or would it be simpler if I just cancel my Shopify account and in a few days open a new one?