If you visit the site, click on the globe or lock icon in the address bar. From there you can clear cookies, local storage (shown in the Cookies dialog) and site settings. Sorry, nothing for cache or history.
@sgunhouse I wasn't talking about My Flow communicating with/through Sync, I was talking about My Flow toggle button not being stored as part of Settings within Sync feature. If you install Opera on one computer and flip My Flow to OFF, install Opera on another system and Sync it with first one, My Flow will remain enabled on second system despite being disabled on the first one.