This has been occurring with customers that are using the Chrome browser. We are currently unsure of what Chrome setting is causing it, but TurboTax Online saves your return after each screen in case one were to get disconnected accidentally. So you can go a neutral screen where you are not inputting any data, and then close the browser.
If the problem continues with your Chrome browser, try clearing your cookies and cache or you can try another browser such as IE or Firefox (Firefox works best with TurboTax).